First I would open a support net service request for further assistance at https://supportnet.mentor.com/
Alternately, you might try disabling "enable concurrent design", close the project and reopen. After correcting the issue enable conncurrent design again.
Can you copy the schematic page into a new block, in the navigator, right mouse button on the page and copy, and then in the block section of the navigator, paste. (you may need a new schematic block to paste into)
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Concurrent design was already disabled.
SR is # 2856553387
Meanwhile I tracked it down to an invalid handle, belonging to the said bus.
A temporary workaround is to mark the bus, then cut & paste it.
Nevertheless we never experienced a similar problem in all the years