Hi,
Here are some workarounds for this issue. This may help you.
1) When XsnLocation or XmlLocation is not specified correctly (in
XmlFormView).
2) In case of SecurityException thrown by the form.
3) After a browser window have this issue, it continues to have this
issue. (need to re-open in new browser and clear cache to browser
cache conflict)
I'll also recommend you to look into sharepoint logs. The event view
log is minimal and does not provide you lot of information.
1) Enable "verbose" logging on sharepoint server: central admin ->
operations -> diagnostic logging -> select category to "all " and
trace log to "verbose".
2) Goto C:\Program Files\Common Files\Microsoft Shared\web server
extensions\12\LOGS and open the last updated log file.
3) check for error related forms server.
Finally, if the infopath form has code-behind file, then debug the
infopath form using visual studio 2005.