Hi,
okay, then unfortunately for me it sounds like a “normal” behavior of a priority based multitasking system like AR.
I assume that the FTP server process has a higher priority inside AR than the OPC UA process, and therefore a running FTP server operation gets/consumes the “remaining processing time that is available for low-priority programs” instead of the OPC UA process (“low-priority” in this context means more or less “everything that runs with lower priority then for example interrupt handlers, IO scheduler, all taskclasses, a.s.o.”).
But as I said, I can’t swear it: because don’t have access to hardware right now, I can’t proof (for example by doing a profiling), if it’s really that context I assume.
I recommend to contact the support team of your local B&R subsidiary to dig a bit more into deep.
Best regards!