When closing Xolox 1.4 build 474, I get:

Exception EAccessViolation in module XOLOX.EXE at 0004F497.
Access violation at address 0044F497 in module 'XOLOX.EXE'. Read of address FFFFFFFF.

Using Win98, ZoneAlarm Pro 3.1.

This only seems to happen if I have a file downloading when I exit, though I'd have to test a bit more to be sure.