Thanks for all of your replies!
Luckily I've found a solution myself by fiddling around:
- In the left sidebar where the folders etc. are listed I hovered over the "title" (mail address) of each account. A wrench symbol appears.
- After clicking on the wrench symbol, I chose "IMAP folders"
- Now this finally seemed to trigger Opera Mail to connect to the IMAP server again (in order to download the list of folders, I guess) because as soon as I did this, my new mail for the said account started to pour in
- Repeat 1-3 for the other accounts
- Everything seems to work as it did before now. If I click on the Check/Send button I also have the status bar appearing again
I'm very happy that this could be sorted out because I'm dangerously dependant on this stuff working...