To sum up what I've found so far and perhaps close the thread:

The client's site problems were resolved by upgrade to FB 1.5.3. I
don't actually think that the crashes were caused solely by older FB
version, because we never had problems like this with 1.5.1 on other
sites and I'm not able to reproduce them on FB 1.5.1 either. Since
I'm not in direct contact with the customer's technical staff, i can
just assume there may have been other changes to software or hardware
of the server's machine.

As for the "can't find shared latch" error message - I spent quite a
while trying to intentionally rise the error - with no success at
all. I tested concurrent reads and writes on "short and narrow" table
using long-lasting transactions, which I originally supposed to be
the source of problems, but neither 20+ readers nor 20+ writers on
the table (and possibly the same page) caused an error or server
crash. Now I suppose that the error was just a side effect of the
server crashes, obviously just on some conditions hard to reproduce.

As a result, let's leave the message in a class of "(almost) can't
happen" errors. Should you ever come across it in your log file,
throw away the machine and try another one.

Regards,
Michal