It is not a ground shaking issue, but probably would be best to handle it more gracefully.

Initially mentioned while doing install using datalad. Account/permission is required to access this particular repo, ask Canadians for access if you don't have it yet Joey. credentials I guess got asked for and cached by git upon initial invocation, so upon subsequent calls didn't ask for any:

$> datalad install https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids
[INFO   ] Cloning https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids [1 other candidates] into '/tmp/Coffey-mri-bids'
[INFO   ] fatal: bad config line 1 in file /home/yoh/.tmp/git-annex96493-5.tmp
[INFO   ]   Remote origin not usable by git-annex; setting annex-ignore
install(ok): /tmp/Coffey-mri-bids (dataset)

which boiled down to that message being spited out during git annex init which samples the remote, but fails to download the config and gets instead a redirected html page:

$> git clone https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids
Cloning into 'Coffey-mri-bids'...
warning: redirecting to https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids.git/
remote: Enumerating objects: 398, done.
remote: Counting objects: 100% (398/398), done.
remote: Compressing objects: 100% (282/282), done.
remote: Total 398 (delta 53), reused 393 (delta 48)
Receiving objects: 100% (398/398), 34.97 KiB | 795.00 KiB/s, done.
Resolving deltas: 100% (53/53), done.


$> git -C Coffey-mri-bids annex init --debug
...
[2019-11-27 19:27:01.341315979] Request {
  host                 = "git.bic.mni.mcgill.ca"
  port                 = 443
  secure               = True
  requestHeaders       = [("Accept-Encoding","identity"),("User-Agent","git-annex/7.20190819+git2-g908476a9b-1~ndall+1")]
  path                 = "/bic/Coffey-mri-bids/config"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}

[2019-11-27 19:27:01.90016181] read: git ["config","--null","--list","--file","/home/yoh/.tmp/git-annex228094-5.tmp"]
fatal: bad config line 1 in file /home/yoh/.tmp/git-annex228094-5.tmp
[2019-11-27 19:27:01.913302324] process done ExitFailure 128

  Remote origin not usable by git-annex; setting annex-ignore

$> wget -S https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids/config
--2019-11-27 19:29:25--  https://git.bic.mni.mcgill.ca/bic/Coffey-mri-bids/config
Resolving git.bic.mni.mcgill.ca (git.bic.mni.mcgill.ca)... 132.216.133.92
Connecting to git.bic.mni.mcgill.ca (git.bic.mni.mcgill.ca)|132.216.133.92|:443... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 302 Found
  Server: nginx
  Date: Thu, 28 Nov 2019 00:29:26 GMT
  Content-Type: text/html; charset=utf-8
  Content-Length: 109
  Connection: keep-alive
  Cache-Control: no-cache
  Location: https://git.bic.mni.mcgill.ca/users/sign_in
  Set-Cookie: _gitlab_session=8a4f8d5569636004aaebfb73588a2d53; path=/; secure; HttpOnly
  X-Request-Id: xTcSyu4H36
  X-Runtime: 0.071681
  Strict-Transport-Security: max-age=31536000
  Referrer-Policy: strict-origin-when-cross-origin
Location: https://git.bic.mni.mcgill.ca/users/sign_in [following]
--2019-11-27 19:29:26--  https://git.bic.mni.mcgill.ca/users/sign_in
Reusing existing connection to git.bic.mni.mcgill.ca:443.
HTTP request sent, awaiting response... 
  HTTP/1.1 200 OK
  Server: nginx
  Date: Thu, 28 Nov 2019 00:29:26 GMT
  Content-Type: text/html; charset=utf-8
  Transfer-Encoding: chunked
  Connection: keep-alive
  Vary: Accept-Encoding
  Cache-Control: max-age=0, private, must-revalidate
  Etag: W/"305857ff0ba591a1e4ee7fec83b5687c"
  Referrer-Policy: strict-origin-when-cross-origin
  Set-Cookie: _gitlab_session=8a4f8d5569636004aaebfb73588a2d53; path=/; expires=Thu, 28 Nov 2019 02:29:26 -0000; secure; HttpOnly
  X-Content-Type-Options: nosniff
  X-Download-Options: noopen
  X-Frame-Options: DENY
  X-Permitted-Cross-Domain-Policies: none
  X-Request-Id: MHFi7Yjxe82
  X-Runtime: 0.063359
  X-Ua-Compatible: IE=edge
  X-Xss-Protection: 1; mode=block
  Strict-Transport-Security: max-age=31536000
  Referrer-Policy: strict-origin-when-cross-origin
Length: unspecified [text/html]
Saving to: ‘config’

config                                                       [ <=>                                                                                                                              ]  13.19K  --.-KB/s    in 0s      

2019-11-27 19:29:26 (89.1 MB/s) - ‘config’ saved [13505]

$> cat config 
<!DOCTYPE html>
<html class="devise-layout-html">
<head prefix="og: http://ogp.me/ns#">
<meta charset="utf-8">
<meta content="IE=edge" http-equiv="X-UA-Compatible">
<meta content="object" property="og:type">
<meta content="GitLab" property="og:site_name">
<meta content="Sign in" property="og:title">
...

I guess the problem is multi-faceted:

  1. in case of authenticated http remote, git caches credentials, but then git annex tries to download file directly (instead of somehow via git), it could not "sense" that remote to be a valid annex and/or get files from it.

You can try with this simple one -- user "demo", password "demo":

$> git clone http://www.onerussian.com/tmp/secret-repo/.git
Cloning into 'secret-repo'...
Username for 'http://www.onerussian.com': demo
Password for 'http://demo@www.onerussian.com': 

$> git -C secret-repo annex init
init  (merging origin/git-annex into git-annex...)
(recording state in git...)

  Remote origin not usable by git-annex; setting annex-ignore
ok
(recording state in git...)

although remote is a proper annex, indeed git annex cannot use it since does not authenticate as git does. So even though the error message is not incorrect, I would say the situation is suboptimal

  1. if remote server instead of just returning 404 or 403 error code (as eg github seems to do in similar cases of non-authenticated access) instead redirects to some login page, annex feeds that page as a config to git, ignores the error message and just marks that remote as ignored for annex, while leaking that obscure "fatal" error message from git.

IMHO, ideally 1. should be addressed properly (authentication), and for 2. annex should spit out some more sensible message ("git failed to parse a config file fetched from the remote X. Please inspect it at this /path/config"), so keep that file around for debugging. As it is now I had to dig quite deep to figure out WTF is going on.

git annex 7.20190819+git2-g908476a9b-1~ndall+1 and the same with bleeding edge 7.20191114+git43-ge29663773-1~ndall+1 (probably that commit is the one with my patch for stricter git versioning, so use the count of 42 ;))