I have seen that error recently in a support case. That resulted in issue CMS-10293 that was released in 11.5.3. If you have the stacktrace (should be possible to capture in log) I can probably determine if you encountered the same issue as in the support case. Otherwise you can just try to upgrade and see if it solves your issue.
I can't copy or import pages from a specific node in the Episerver page tree. Export of pages works fine, but when importing I get the message in "Exception: Value cannot be null. Parameter name: source". Same thing happens when trying to copy any of the pages in that node.
It's not limited to a specific page type, and other pages of same page types from other parts of the page tree works fine. We can reproduce it on any environment. Nothing shows up in the logs, Visual Studio doesn't break execution when debugging. I have no idea where this error happens :-(
Error message when trying to copy a page:
Episerver CMS 18.104.22.168