Random imposibility to login/operate

Steps to reproduce
1. Going to the owncloud web
2. Input username and passowrd (any username)
3. Click on the authetication arrow

Expected behaviour
Usual login, files displayed.

Actual behaviour
Sometimes the login screen does nothing. Sometimes, while operating through files, the list dissapear.

Server configuration
Operating system: Debian Jessie, up to date.
Web server: Apache 2.4.10-10+deb8u8
Database: Mysql 5.5.54-0+deb8u1
PHP version: 5.6.30+dfsg-0+deb8u1
OwnCloud version (see ownCloud admin page): 9.1.4-2.1, from Owncloud repository
Updated from an older ownCloud or fresh install: Updated from owncloud 7 (debian packages); no version skipped.
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption): None.

ownCloud log (data/owncloud.log)

{"reqId":"ckk3+5\/xSLVdZ8yLc08e","remoteAddr":"","app":"no app in context","message":"Current user is not logged in","level":0,"time":"2017-04-18T17:34:25+02:00","method":"GET","url":"\/owncloud\/index.php\/avatar\/admin\/128","user":"--"}

Integrity status for oC9+

No errors have been found.

Found this: https://central.owncloud.org/t/owncloud-login-button-wont-work-and-files-after-login-wont-shows-up-also/1707; deleting the "disabled="disabled"" code lets me in but, as the original poster, no files (and no menu is working). Also found two other topics but, as a new user, I'm only allowed to post one.
My server is at home, no VPS, no cloudfare, etc. Everything was working fine with Owncloud 7 packages.

I think your posted link contains nearly all needed and known information about your issue. Most likely an environmental issue where it is nearly impossible that we users in here can help debugging it remotely.

As you can read in the linked thread the same can happen at home if your webserver config is broken (e.g. some strange configs or modules enabled).

ownCloud 9.x switched the WebGUI to WebDAV so this is further showing that there are some issues in your webservers config.

What's currently missing in this list is:

  • Own custom theme
  • Broken 3rdparty apps

But the logfiles of your webserver, php, the data/owncloud.log as well as the browsers error console might tell you more.

Well, kind of embarrasing... but SOLVED!!

I had had changed

ModPagespeed Off

to "On" in .htaccess. Why? Don't know, sometimes I break things....
As soon as I changed to "On" and restarted Apache, everything is working fine again.

Thanks for the help, kljhlkhglklfgh!

You shouldn't use mod_pagespeed at all as it has shown various issues with ownCloud (e.g. by messing around with javascript and similar) so its recommended to remove this 3rdparty module from your webserver.

Thanks! Will do right now.