We've 2 calls to that function that only happen during a critical user store error. Will go ahead and fix them now, but they likely aren't the cause as there's probably other Joomla 3 extensions installed on a Joomla 4 install. Do you've any 3rd party User plugins installed in Extensions > Plugins?

forge.joomlapolis.com/issues/8914

Ok, latest CB build release has removed our remaining stderr usages entirely. I'm unsure if this will solve your problem though as could be from another extension acting on Joomla user store.

Are you still getting that error after having updated CB? If so then it's not coming from CB as we only had 2 calls to stderr and both are now removed entirely (we also removed that function as well since it's gone in J4).

Suggest checking your server error log and seeing if there's more details regarding that error as without a callstack that tells you what file and line number that error is happening on there's nothing anyone can suggest. If server error log isn't showing it be sure you've triggered the error with Joomla debug mode and maximum error reporting configured in Joomla global configuration so errors aren't suppressed.