You might have heard about this issue
When an iNotes user opens their mail file on a Domino 9.0.1 Fix Pack 7 server, a “Formula Error” appears. This issue is known to occur on servers with non-English OS Regional Settings.
from different sources or maybe ran into it after installing FP7. Moments ago, IBM Support published an update to the technote mentioning that a fix is coming soon:
“by end of September at the latest”
IBM Support also documented three possible workarounds, from reverting back to a previous FP to changing regional settings or numbering format.
Go read the full story in the published technote