I'm having the same problem mention in this article below, but the author doesn't give the answer or explanation how he fixed the issue. My digital signature works on thje client but not in the browswer. Please help.
http://sheetal-d.spaces.live.com/blog/cns!237C3DEA7120098B!634.entry?sa=477213976
Hello Michael,
To digitally sign the form in browser enabled form you need ActiveX controls.Look at the below link:
http://office.microsoft.com/en-us/sharepointserver/HA102884101033.aspx
I've done all of that and I still get an error. The error says An error has occured. Please try again.