What happens when you try to save? Any error messages?
I suspect the problem is with the Topaz plug-in. There have been significant changes in Reader 10, many security-related, so I would first check with Topaz to see if they have an update or are aware of compatibility problems.
I ran into that issue and here's my 'fix'. Wasn't Topaz. I set up forms for others to sign with a Topaz sig pad and here's what I've discovered. I have 3 steps in my files. Create the form as a Template (with the form Edit open). Then File, Save As PDF and rename as Fillable. Then 'Close Form Editing' and then Save As Reader Extended PDF, Enable Additional Features and name the file FILL. So my files would look like:
Then I send the FormnameFILL to the users. When they open the form and start filling out the form, BEFORE they sign the MUST Save the form FIRST. Sometimes it would require saving after each signature.
Then if they get the above error, just SaveAs and rename it Formname2FILL or whatever and try again. Seems I've run into that if the form was emailed or opened previously.
It is strange when you get that error and you don't have any other pdf forms open.
I think there must be something running in the background that hangs with Acrobat.