Finder has trouble mounting folders with special characters
Without having to learn the entire illegal character list for every OS, it's best to train people to an easy subset of these rules…
Alphanumeric only. No umlauts, etc. No spaces, use underscores.
You could probably make a call on Capitals, most OSes are case insensitive, so it shouldn't matter.
Don't let them use edge cases &amp; you won't have to fix them;) This is especially important if you have multi-platform access, as illegal characters on one OS may not be on another.
I'd make a rule on replacing umlauts too, decide whether Müller should become Muller or Mueller &amp; stick to it - that way people will not need three guesses to find an otherwise known name.
Using underscores with no spaces just makes paths simpler - no-one has to guess whether they need folder name folder\ name folder%20name or &quot;folder name&quot; to find something, it's always folder_name.
% is a URL escape [encoding] method as seen above, %20 means space - best to be avoided for names in URLs.