I just found a painful bug which I've been trying to fix for weeks!!!
Every now and then my DB2 Win2K database would just stop working properly, it would give an error SQL0440N and say the upper function was unknown. I had not changed any settings at all, it just happened all of a sudden. I found a few discussions on similar errors but they had to do with installations and setting funcpath. After a few reboots the database would start working again.
I eventually stumbled across the answer, I was doing some date testing and setting the system date back a year. If I rebooted with the date of a year ago DB2 would not work until I set the date back to this year.
This may be obvious to DB2 experts but for me, new to DB2 it had me stumped