What's happen

18:15:06 Fuzzy Checkpoint Completed: duration was 1 seconds, 429 buffers
not flushed.
18:15:06 Checkpoint loguniq 25630, logpos 0x2606a0

18:20:07 Fuzzy Checkpoint Completed: duration was 0 seconds, 136 buffers
not flushed.
18:20:07 Checkpoint loguniq 25630, logpos 0x29b51c

18:20:12 Logical Log 25630 Complete.
18:20:13 Logical Log 25630 - Backup Started
18:20:13 Logical Log 25630 - Backup Completed
18:20:13 Assert Failed: No Exception Handler
18:20:13 Informix Dynamic Server 2000 Version 9.21.UC3
18:20:13 Who: Session(2159834, statist@dbinter1, 18539, 688637964)
Thread(2790282, xchg_1.0, 2908f238, 3)
File: mtex.c Line: 405
18:20:13 Results: Exception Caught. Type: MT_EX_OS, Context: mem
18:20:13 Action: Please notify Informix Technical Support.
18:20:13 stack trace for pid 3842 written to /tmp/af.9772792c
18:20:13 See Also: /tmp/af.9772792c, shmem.9772792c.0
18:20:57 mtex.c, line 405, thread 2790282, proc id 3842, No Exception
Handler.
18:22:04 Fatal error in ADM VP at mt.c:11380
18:22:04 Unexpected virtual processor termination, pid = 3842, exit = 0x100

18:23:04 PANIC: Attempting to bring system down

Did this happen during a stored procedure? If so and you can recreate the issue, see how many threads the process starts. If its more than you expected, then do this:

set pdqpriority 0;
update statistics for procedure <stored procedure name>;

Also, if it is a stored procedure, then you need to check the code and if it has other stored procedures nested within it then you need to do the above for those as well. I ran into this issue on IDS 9.40 when I had a programmer updating statistics for procedure with a PDQ level of 100 and it took me forever to track down what was going on.