The source of the error has been found, please take a look at #14229 for the fix.

This error has been reported quite a few times on the forum, so I'll create an issue.

What is known:

• It does not depend on PHP version
• it often occurs with 3.9.0, but somone already reported this for 3.8.1
• it does not occur on any of my Matomo instances
• reverting Enable fingers crossed handler via INI config and show backtrace in logs/archive api output #13923 fixes the issue (see Notice - unserialize(): Error at offset 0 of 53988 bytes #14229 (comment))
• it only occurs during archiving (if you see the error somewhere else, please comment)
• ???

https://forum.matomo.org/t/empty-or-invalid-response/31848
https://forum.matomo.org/t/empty-or-invalid-response-when-core-archive/32106
https://forum.matomo.org/t/error-making-archive/32110