Group folder sometimes not "present" for some users

We’ve got a setup where there’s a lot of group folders, including nested. One of these group folders now occasionally disappears for some users. The group folder in question is a parent folder with quite a number of nested group folders, and is also unique in the setup by having a “,” within the folder name.

The user responsible for the group folder who gets most of the reports thinks that most and maybe all the reports relate to users who have just logged on - because the folder contains various company forms so people have just clicked a link to get to a form, logged into NC and then get an error like
Sabre\DAV\Exception\NotFound File with name <insert correct file name, group folder name and path> - at the same time that other people have been able to click the link and get the file.

There are related reports of people being told where to find some content by reference to the group folder name, logging in and then that group folder not being visible. 2 actions seem related in terms of resolution; one is log out and back in and the other elapsed time i.e. early reports seemed to conclude that logging out and back in resolves the issue, but more recently (when people have immediately attempted this as a “known fix”) it appears time, on the order of 15 minutes, is a, or even the, factor.

Any ideas for a fix?