I think this is caused by a issue I fixed here: https://github.com/nextcloud/server/pull/3825
I plan to backport it to stable11 as soon as it merged. For the issue we discuss here, I think this particular change will fix it: https://github.com/nextcloud/server/pull/3825/commits/1f1e36ac6560161ed6c26b8dbe980bfc62733581#diff-ec69af4ceadfc4207fcfd3373d0b9338
If you feel comfortable enough you could apply this changes to all the involved servers. Remove the trusted servers and add them again. (You only need the changes for apps/federation/lib/Middleware/AddServerMiddleware.php and can ignore the changes to the test file)