Hello, welfi! How about your problem? Do you solve it? I have the similar one.

My case:

I have old PC (WIndows XP, WinCC 6.0) and new one with migrated project from old PC (Windows 10, WinCC 7.4) both with 5611 cards, sequently connected by mpi connection to the S7-300. New PC is the last in mpi line with terminator is ON on its 5611's connector. 

Several times a day on both PC connection with S7-300 is lost for about 5 secs, on old one all IO fileds are just grayed (according usual state display behaviour in WinCC) but on new PC also popping up dialog box with error "A SYNI error occurred in \Device\s7opciax#1: Bus short circuit or wrong transmission rate or partner device not powered up" that need to be clicked for remove it from screen.

Could this to be solved?

------------------------------------------------------------------------------------------
Split from cp5611 and mpi connection.

Of course info I provided is very brief. But problem surely not in "unique MPI addresses used for each device on bus and same comm speed set". All is working except some random moments once or frequently per a day. Also the matter is that problem started after adding second (new) PC at the end of mpi segment. If I do switch terminator ON on old PC (thus cut off the rest of mpi line with new PC), then problem is not appeared after that.

Sure I will try to provide more info about my situation. Thank you for answering!