This error may occur when saving a new matter. It has to do with the client or file name containing restricted symbols.
These restricted symbols are: \ / : * ? " < >
This restriction is only in place if you are using the document management app integration (Dropbox, Box etc) as a file path needs to be created, and these symbols are not allowed.
To resolve, edit the matter or client name that contains such symbols, remove them and update.
You will then not have any issue saving your matter