I was getting this error as well - and a similar one when trying to insert a single page.
Trying to delete the cache did not work at all - so I decided to take a closer look at the code on the page. The code I was working with came from a different developer, therefore I figured something might not be right.
Turns out it was the "DOCTYPE" line on the page causing the problem.
The one on the page read
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//en" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
All I had to do was capitalize the LANGUAGE and the plug in worked. (ie: "en" to "EN")
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
This seems to be a very odd thing to cause an entire extension to stop working!
I'd love to hear a response from someone at WebAssist as to why a lower case language would cause such problems.