One Email account is bringing down the whole program.
Could be that account that’s crashing might be possibly corrupted in eM Client, so causing it.
If it’s an IMAP, Exchange, Office 365 or iCloud account, suggest to try removing and re-adding that specific account if you haven’t tried already. That would normally fix it if it’s just one account.
On the Mac, go to “eM Client / Accounts” in the menu top left. Then click on your account on the left side and click remove / delete along the top. Then click add account using either the automatic email wizard or manual setup depending on your specific mailbox setup.
If it’s a POP account, create a folder down in eM Client “Local folders” at the bottom left called eg: Old Mail, and then drag all your current POP mail complete with your mail folders down under Old Mail.
Then go to “eM Client / Accounts” and remove your old POP account. Then click Add Account / Mail / Other & choose POP. Once you then have your POP account added back in, drag / move all your Old Mail from local folders backup up to your new Mail account.
Lastly if removing and re-adding your account works, then make a backup of eM Client via the “File / Backup” menu. You can see when the backup is complete in “Show Operations” via the dropdown on the right of Refresh top left.