Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey 6.1.5
Own server or LimeSurvey hosting: own server
Survey theme/template:
==================
I am also encountered with the world famous "session not writable" error message. Whatever I did I was not able to get rid of it, even if the session file was created on the file system and some php session test were successful.
Apparently everything was fixed earlier, and don't know what step did help, but it was not working until I restarted the installation. So neither the page reload, nor the check again button was sufficient.

Just FYI.

So this is not a question, just to give some feedback for others, I assume?

Yes, just a feedback if someone searches for the message, so can have yet another hint.

However if it helps, I can provide more information, just ask.