In version 9.1 SP1, you cannot add the special characters \, /, *, >, <, | to a content title. or, if your content was created on version 8.5sp3 or older and already contained special characters, you cannot edit and save it without removing the special characters from the title.>
If you install 9.1 SP1 CU2 or later, this behavior is changed to allow you to use the special characters: '\' and '/', which were available in 8.5SP3. The other special characters are still not allowed.
You can locate the appropriate download here:
Under: Ektron CMS / Version 9.1 / 9.1 SP1 / Ektron 9.1.0 SP1 Site Cumulative Update
Please be sure to read how to apply a CU here: /KB/10132/