I had the same situation as poynteca while updating from flex viewer 3.0 to 3.1
my findplace inside of my custom header was giving me the following errors:
"widget manager error #2036"' on my flash builder but on the browser would report "error #2035"
another thread let me know that error #2035 was URL Not Found, I did a search on my findplace.as for url's and found the path:
url: "widgets/InfoTemplates/InfoPopupWidget.swf"
I checked the modules list, for whatever reason the flex viewer 3.1 I downloaded didn't have registered the modules as previous viewers:
src\widgets\InfoTemplates\InfoPopupWidget.mxml
src\widgets\InfoTemplates\SimpleInfoWinWidget.mxml
I added both of them through the project properties and the error went away
may be you can follow the same process to solve your error