What does reproducing problems mean?
You do "a" in an app, you do "b" in the app and get a crash or see a bug. You do "a" again followed by "b". If you get the same behaviour again you have reproduced the problem.
I sometimes get emails with "I tried do archive 3 times in Mail Archiver. But every time I got a crash".
In general a reproducible problem is waaayyyy simpler to fix compared to a non-reproducible problem. We call that having "half the rent" in German.
After you have reproduced the problem then I need to reproduce the problem. That's sometimes a bit tricky because I need also to have the same situation as you. Both you and I have different situations which may affect reproducing the error.
Mail Archiver and the permissions problem
In Mojave you need to give an app extra permissions when the app wants to do an AppleScript. Mail Archiver uses AppleScript for AppleMail, Evernote, Filemaker and Outlook. The security feature is described in this blog post. Now and then I got reports that even with the permissions set in Mojave there was an error message that the permissions were declined.
I wasn't able to reproduce the problem - not after checking and rechecking everything. Then I got the correct hint: AppleMail has to run BEFORE the permissions are checked. As developer my computers are always on. AppleMail is ALWAYS running. With that little fact I was able to reproduce the problem in a minute. It took a couple of more minutes to add a check if AppleMail was running or not. And if it's not running AppleMail is now started.