About month ago, in different customer sites, message
Index tag ... is corrupted. Plese rebuild index starts to occur.

Reindexing removes this message for a while but it occurs again and again.
Users can ignore this message and in this case last added records are not found
if index search is performed by this index.

VFP9 application is running for years without this message.
It started to occurs about month ago in 3 different customer LAN networks.
In one site LAN server is windows 2012 server so opportunistic locking cannot disabled probably.

How to fix or diagnose the isse ?
Is there some windows automatic update which caused this ?