If the index expression can create index keys of different length, you may get this error. What's the index expression?

>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 ?