How to replicate:

• Open the dialog to change the validity of the certificate for one of your own keys.
• Click in the date field and change the last digit so your date is the current date (which is not acceptable to Kleopatra, you can choose an even earlier day, too)
• Click on the drop down triangle
• Click OK in the upcoming error window (or close the window with click on the x)

-> Crash (after about a second)

Debugview only gives the line:

<process started at 16:47:21.671 has terminated with 0xc0000005 (EXCEPTION_ACCESS_VIOLATION)>

The error does not occur if you click OK after entering the too early date. Then the error window comes up, too, but clicking OK then does not result in a crash. If you click on the triangle after that (without changing the date again), the error window will not come up and there is no crash.