Details on specific pages attempted: https://en.wikipedia.org/w/index.php?title=Wikipedia:Requests_for_page_importation&oldid=1293751115#Import_of_de:Liste_der_K%C3%B6sener_Corps_to_User:Alrael/List_of_K%C3%B6sener_Corps

Note: same exported XML files WORKED as expected when importing to testwiki:
https://test.wikipedia.org/w/index.php?title=Special%3ALog&logid=426920

It was almost certainly a file size issue (the relevant XML files are 18,015KB and 11,650 KB, respectively), the former being the largest file I've ever imported on Wikipedia. For what it's worth I got both those imports to work, but they took a while (I had the importing page opened for several minutes each time) and I also got a Wikimedia error.

I just tried a much smaller import from the August 2001 database dump at Letterboxing (hobby), and it worked completely as expected.

I've therefore marked this task as resolved.

As I noted ,the same process worked on testwiki, so this seems like an actual production processing error (likely some sort of timeout). If the root cause is "filesize" - then there is still a problem: the failure error is incorrect.

OK, re-opened it (marking it as resolved was probably beyond my purview here). But to me this is a normal failure mode for things that fail due to too much info being asked for ... like resource-hungry contributions queries ... and, back in the old days, resource-hungry deletion/undeletion requests (which are now dealt with by the replication lag trigger and the 500-edits-at-a-time thing in Special:Undelete). Have I just spent way too much time abusing the poor old servers?

I ran into repeated import timeouts when running https://www.mediawiki.org/wiki/User:Flow_cleanup_bot. But I was importing pages even larger than that, and found about 40MB to be the sweet spot when importing things there - any larger would end up failing.

I would be inclined to close this as a duplicate of T384167

T384167 seems like a proposed solution, this is the report of an actual production error. AFAIK, no one has yet investigated this actual production error - which can then be associated with a bug/defect - or become the report of the defect. This has been going on for a long time without being addressed.