14000 lines of Fatal Errors a day

My php error log gets spammed with this:
[28-Sep-2020 10:48:13 UTC] PHP Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed in [no active file]:0
Stack trace:
#0 [internal function]: session_write_close()
#1 {main}
thrown in [no active file] on line 0
How do i figure out what piece of code causes this?

jero replied on at Permalink Reply
When PHP saves data to a session, it has to serialise (convert into a special string) any data in the session variables so that they can be saved. From what you're describing there, it appears that you are somehow getting an object or other code of some sort set in to the session, and that code contains an anonymous function (a closure) which can't be serialised. Have you got any bespoke packages or themes etc that might be responsible for this? They are the first place I'd look to try and resolve this.
dimger84 replied on at Permalink Reply
This is a bug in concrete 5.8.4. There is already a fix merged in github repository and it will be released in version 8.5.5.

see here:https://github.com/concrete5/concrete5/issues/8791...
and here:https://github.com/concrete5/concrete5/issues/8824...