"Index not valid"

I've a known Problem but I still can't solve it:
I'm running an app in a network, a few users are accessing the same
tables simoultanously.
Now, after some time spent, and when i already post & edit a lot, an
error occurs:
(I translated this from german, so it isn't right)
" Index not valid anymore, Indexname_abc "
( Original: Index nicht mehr gltig ...)

It has to be something about the simoultanous access so the users crash
my beloved index.
I have to solve this at runtime and i'm not able to change much of the
program (in fact, this should be final testing), so if somebody has a
short solution, i'll be thankfull.

I haven't the possibility to search the newsgroups very often, because
I'm a student and
I use a dial-in-pay-yourself-stupid-account (it takes 10 Minutes to
download all headers of
the group). So, PLEASE, PLEASE leave your answer
in my mailbox: " l...@i-node.at " (and in the newsgroup).