There is one rule that's valid for every data transformation: Keep a back-up of your original files. ALWAYS!
In general nothing can happen with your emails, except when you use "Move to Trash". So use this feature with care.
There are two kinds of problems in Mail Archiver X. Either there are problems during archival or with the Email Viewer. Or the database is corrupt.
First you notice it. An application crash is different from a hard crash, where the OS wants to send information to Apple. Here is a screenshot of a manufactured application crash:
Add a description what the problem is about and what you did before.
Mail Archiver records some data what it has been doing. This is written into "Session Log" files in the Application Support Folder. The files are deleted after 30 days. These files are sent with the email.
You can view the session logs when you select the menu item Help/View Session Logs.
You can also send an email at any time about an error without having the application crashing with the menu item Help/Send error report.
If for some reason you can't send an wmail you can get to the session logs in Finder with the menu item Help/View Session Logs in Finder.
The menu items of File/Database gives you the possiblity to get information about your database and its problems.
When you open an archive a file will be created with the name "Mail Archive.journal" if the name of your archive is "Mail Archive". This is used to protect the database from data loss in the case of a crash. Just ignore this file. Similarly, the file "Mail Archive.DBIndex" is a helper file that contains the index for the database.
Whenever the application crashes when accessing the database there is a risk that the database is damaged. When you restart Mail Archiver and it takes a while to open the last access archive then Mail Archiver tries to restore the data. But this doesn't work in 100% of all cases. What can you do if you get an error message after archival? This is how the error message looks:
If you ignore this message it's very likely that you are going to lose data!
Go to the menu item Database/Dump.
Next you will see a dialog to save your dump data somewhere:
You will need to wait a while. Also, you need to know that resulting file will be HUGE so select a location that has lots and lots of free space. Afterwards close the open archive. Then go to the Database menu again and select Database/Load Dump.
You need to select your file that was just created.
A new database will be created after you have selected a location and a name. Then the last step is done - the data is loaded.