Page not found - but it exists?

Forum members have recently been getting an error on one site for two directories and their contents: "blog" and "activity".

The error is:

"Page not found
The page you are trying to view does not exist or you do not have permissions to view it"

In the admin area there was a message:

"blog (guid: 3) cannot start and has been deactivated. Reason: FilesystemIterator::__construct(/Users/folder1/data-dirs/folder2/caches/localfastcache/elgg_local_system_cache/Files/ce/46): Failed to open directory: No such file or directory"

When I look at the directory, it is there and has permissions like any other directory.

I cannot figure out what went wrong. Any ideas?

I use the 5.1.4 version of Elgg, running on MacOs latest, with nginx and mariadb. I have other forums which are configured identically and do not have problems.