There is no problem with the code. Even <script></script> gives the same error. If I on the other hand just write "Test" it works! So something with <script> is making it brake.
Saving should not be a issue, since it is done entirely by Wordpress's widget API. If the problem occurs only if you add a <script> tag then post the code here for review. Another reason is that, your server should be good while saving. If it is down then you it wont be saved.