I was a little in doubt about where to post this, because it's basically an interaction.

What happened was that since last Wednesday my inbox folder stopped syncing. Only my inbox folder, everything else synced fine and I could send mail.

What fixed it was running akonadictl fsck

This revealed missing external files and unreferenced external files. After this, KMail synced again.

So one might argue this isn't a bug in KMail, but I actually think it is for the following user: No end user should ever have to run akonadictl fsck in a terminal. KMail should react to something going wrong in akonadi that could be related to missing external files and run akonadictl fsck for you. I consider it a bug because it effectively broke my ability to receive e-mail.

Here's what I got on clean-up:
Cleaning up missing external file: /home/christian/.local/share/akonadi/file_db_data/25/78325_r1 for item: 38197 on part: 78325
Cleaning up missing external file: /home/christian/.local/share/akonadi/file_db_data/51/143951_r0 for item: 70482 on part: 143951
Cleaning up missing external file: /home/christian/.local/share/akonadi/file_db_data/53/143953_r0 for item: 70483 on part: 143953
...
Found unreferenced external file: /home/christian/.local/share/akonadi/file_db_data/76/144176_r1
Found unreferenced external file: /home/christian/.local/share/akonadi/file_db_data/25/78325_r2
Found unreferenced external file: /home/christian/.local/share/akonadi/file_db_data/95/147195_r1
...