Alright, "Rebuild and Rescan Databases" did the trick! Thanks for the input.

So, rescanning the DB didn't change anything. Armory *looks* fine, but my transaction is still missing and when I try to send a new transaction I get "missing inputs" error. Does that make sense and is it directly related to a missing transaction in the history?

Here's the log: https://pastebin.com/euErbsHd

Thanks for your answer!

Ok, I'm trying the rescanning, and if that doesn't help, the rescanning & rebuilding and will report back.

However, it looks to me like my Armory is doing this on every single startup anyway. It takes about half an hour for each start, always rebuilding DB, organizing blockchain, scanning transactions, ...
And when closing Armory (close, not minimize) in 90% of cases, I have to manually kill the ArmoryDB process because it won't shut down on it's own even after waiting for close to an hour.

I guess that's part of my problem? If Armory would shut down cleanly, it should also start up much faster?

I had a BTC transaction stuck with 0 confirmations in Armory 0.96.4, but was able to check on blockchain.com that it went through a long time ago and has 17792 confirmations.

So I followed this advice: https://www.bitcoinarmory.com/troubleshooting/#i-sent-a-transaction-but-it-is-stuck-with-0-confirmations

Sure enough, the transaction has disappeared from my Armory wallet. I expected to see it again in its correct state, after restarting Armory, but it stays gone.
This can't be right? After all, it really did happen and the blockchain knows of it and has thousands of confirmations. How can I get it back to show up in Armory? I guess that means my current balance shown is wrong?

Thanks for any help with this.