I have been sharing folders with a partner for about 3 years on Dropbox without any issues. It's the second time though that I have to send him the documents by email as he can't find the folders any more. He still shows as having full access to them. So from my side I don't see anything strange.
He hasn't changed his email address, me neither. I'm the owner of those folders and no one else than he and I have access to them so no one interfered there. I haven't changed any configuration, everything is exactly as it used to be.
Another co-coworker shares his own folders with him and he's got no problem to find them.
I’m suspecting his Dropbox may be about to exceed its capacity. I’ll ask him to check but I’m afraid he’s not very tech savvy. These are folders I’ve started to share with him some 3 years ago or do and I’m adding files (nothing heavy) regularly. So these are no new folders. He used to see them and now he can’t find them.