All of sudden, when opening an existing InfoPath form instance through IE8, the error below popped up.
Webpage error detailsUser Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB7.4; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.1; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)Timestamp: Mon, 17 Dec 2012 23:07:27 UTCMessage: Can't move focus to the control because it is invisible, not enabled, or of a type that does not accept the focus.Line: 2Char: 499033Code: 0URI: http://SharePointServer/_layouts/inc/Core.js?rev=D9rcLY97a1ECurcRSuOf8A%3D%3D
But, if I open the same form through IE 9+, Firefox or Chrome, then there is no problem at all.
I changed the form a little bit before the test users reported this issue, but I could not figure out what change caused it.
So I decompressed the XSN file to a folder, then compared the "manifest.xsf" and the view XSL file with the previous version, but, no luck.
So, after two days of struggling, in the end, I had to remove the fields from the form view part by part. In my case, a naughty DropDown list control field seems to be the source of pain, and I had to re-create it.
Things are resolved. I guess it's caused by a bug of InfoPath designer.
Hopefully this can save you some time. :-)