Serialization is not allowed
From Joomla! Documentation
When Smart Search is used and the Gather Statistics is enabled an error may appear with the text "Serialization of X is not allowed." where X can be different like Closure or PDO or something else.
Errors reported[edit]
The error has been reported in issue https://github.com/joomla/joomla-cms/issues/38370
Versions affected[edit]
General Information
This pertains only to Joomla! version(s): 4.2.0
What is the cause[edit]
The cause lies in the serialization of the data where the data contains classes that cannot be serialized.
How to fix[edit]
The workaround for now is to turn off Gather Statistics in Smart Search