Logging in to certain servers on certain networks is not possible due to a "Failed to verify username!" error message. The launcher is logged in to a Microsoft account with a valid Minecraft license. Server online mode is active.

Wireshark shows successful TCP & TLSv1.3 traffic to sessionserver.mojang.com during the connection attempt.

Error happens always network-wide on certain networks but only for certain Minecraft servers. There is no firewall or DNS issue - this has been intensively investigated. It is not possible to find the cause on the network by any sort of diagnostic method.

Log output in Minecraft does not exist for this error. Therefore a log output which shows the root cause of this error is mandatory. Also the entire functionality/subroutine that leads to this error should be severely questioned.