iOS Client cannot connect with one user, other userids works

Please help us by providing the following info.

Steps to reproduce
not really reproduceable. Only one user does not work. other users from the same iPhone do work well.

Expected behaviour
User shall login.

Actual behaviour
User does not login.

Mobile client configuration
Client OS (iOS, Android, ...): iOS 10.2
Client version: all clients i tried: OC 3.5.0, nextcloud 2.11 (0011)

Server configuration
Operating system: Ubuntu 16.04
Web server: Apache
Database: MySQL
PHP version: 7
ownCloud version (see ownCloud admin page):
Updated from an older ownCloud or fresh install: fresh
ownCloud log (data/owncloud.log):

Special configurations (external storage, external authentication, reverse proxy, server-side-encryption):
reverse proxy - yes

ok, so the first problem is:
- i cannot find any error messages others than the popups on the client "Server not accessible" - are there any Client-Logs I can download ?
second problem is:
- as it seems to have to do something with my userid, why there are no entries in my server-logs ?
third problem:
- I assume it has to do with some details for my account. As i use a lot of shares back and forth, is there a way to review all my db entries in one place ? e.g. a list of all shares i gave out, and all shares i linked in. including a status-icon whether the external link could be reached or not.

Hi, @scheppi2610

Could you tell us the exact error message that you get when you try to login? We don't recognize "Server not accessible".

Do you have problems with this user account both in Android and iOS clients? What about desktop client, did you try it there? Does it work correctly in the web UI?

Does your user or your password in the account include non-alphanumerical characters? Umlauts, maybe? % symbol?