I found the cause of the problem - but I can't understand the reason.
I have a NUMUPDOWN control to select a number from 0 to 12.
When I removed it and replaced it with combobox - the problem disappeared.
The code for the control (from the design part):
Code:
addnum(portpanel,Hourupdn,95,160,50,2,12,255,255,255,True,True)@
The problem exist even if I mark all the line codes refering to this control in the program.