Thank you both for your suggestions,
I've tried lots of things, but nothing seems to work. Current ticket number is Ticket# 3858652.
Currently the search function doesn't work and it seems to be linked an issue with 'shared notebooks', because when I opened a new notebook and moved all notes there, it worked again. When I shared this new notebook with the team, everything was still good. But as soon as someone else made a change to one of the notes, the problem came back and we couldn't look up anything. We can only search by tag , not by words.
When I exported the notes to an enex file and then imported them again to a new notebook, the search function worked again. When I then moved them to the shared notebook, it all worked up until someone else made changes and then it didn’t work any more for none of the notes.
The Evernote developers are working on fixing it, but would be nice to get some priority since this database really is a daily tool needed for our team to function properly.
Thanks so much for your suggestions,