what else can lead to “Session Manager won’t work”

I use the Session Manager plugin. I have two almost identical installations of the same website, one local for development and the other on remote server for production. Some weeks ago I discovered the message on the local site: “Another plugin is attempting to start a session with WordPress. WP Session Manager will not work!” First I commented out all the lines with session_start() in each plugin – one after another. The message remained. Then I saw that it was caused bei some initial statements like $_SESSION[‘var’] = []; in my own coding. After I had all of them removed, the message had gone.
Then I made a complete duplication of this local installation and loaded it up to the remote machine. All worked fine. But yesterday I discovered, that on the remote server in the backend this message is back: “Another plugin is … Session Manager will not work!”. Ugh! The only difference is that on the remote server Akismet plugin is activated, but Akismet not seems to start a session. And deactivating of Akismet won’t let disappear the message. So I went through the same procedure some weeks ago as I described above with the local version. But I could not find the causing plugin. In other words, the plugin that seems to be causing this conflict won’t start a session neither by session_start() nor by something like $_SESSION[‘var’] = [];.

What else can cause this conflict or trigger this message?

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