Might be a good idea to grab a fresh copy of the zip and re-upload the /inc/ folder

¥

Has somebody uploaded a new copy? I've downloaded it only a few hours ago

No, but sometimes the download is corrupt .... you'd be amazed how many problems a fresh download cures ;)

¥

Not in my case, though. Maybe I'll have to do a complete fresh install.

There are some 1500 files in the B2evolution distro. All need to be transferred to the server through FTP. Unfortunately not all FTP programs check if a file is correctly transmitted and accepted. This may lead to some files being corrupt or missing without a warning or notice being send to the user.
If you use FireFTP there is a box in the left bottom corner telling if and how many files didn't get send.

http://www.blog.hemminga.net/B2evoforum/FileZillaTransfers.png

I am by no means discouraging you from using the software you like, but in an FTP program a feature warning you for failed transfers is useful. ;)

Good luck

Yeah, I suppose you're right. I've used Transmit that does not display error messages. Cyberduck does, but all files were uploaded and b2evo is installed :)

Thanks for your help!