I installed the KeePassXC-Mail add-on according to the instructions in the README. However, instead of KeePassXC, I'm using the original KeePass with the KeePassNatMsg plugin.
The add-on installs just fine and after clicking the "Connect with KeePassXC" button in the add-on settings, I get the usual dialog in KeePass to add the new client to the database. The add-on then shows its connection key in the settings. Thunderbird's error console doesn't show any errors as well:

However, after I restart Thunderbird, a dialog prompting me for the password to the mail server appears. KeePassXC-Mail seems to start looking for the password, but never finishes. It instead keeps showing the message "Loading passwords.." until I enter it manually:

I added the credentials quite some time ago to my KeePass database, with the URL smtp://<myserver>. This worked without any problems with the KeeBird add-on I used previously.
I installed the KeePassXC-Mail add-on according to the instructions in the README. However, instead of KeePassXC, I'm using the original KeePass with the KeePassNatMsg plugin.
The add-on installs just fine and after clicking the "Connect with KeePassXC" button in the add-on settings, I get the usual dialog in KeePass to add the new client to the database. The add-on then shows its connection key in the settings. Thunderbird's error console doesn't show any errors as well:

However, after I restart Thunderbird, a dialog prompting me for the password to the mail server appears. KeePassXC-Mail seems to start looking for the password, but never finishes. It instead keeps showing the message "Loading passwords.." until I enter it manually:

I added the credentials quite some time ago to my KeePass database, with the URL
smtp://<myserver>. This worked without any problems with the KeeBird add-on I used previously.