If the hierarchy separator is not a slash, and there is a slash in the folder name, Claws does weird things.

Specifically, it appears to treat slashes as a hierarchy separator regardless, and when sending messages to the server, converts slashes to the hierarchy separator.

Basically, it looks like it replaces the hierarchy separator with a slash for internal use, then replaces slashes with hierarchy separator for messages to the server.

So if the hierarchy separator is a pipe, the folder "Language|C/C++" would be interpreted as three path elements, "Language", "C", and "C++", and Claws would try to fetch from the "Language|C|C++" folder.

There is an imap server running at nix.synchro.net which supports a guest user (pw: guest) that shows this in action.