I got myself into a bad state a while back where I was unable to open messages attached to other items. It happened while I was monkeying around with this custom form which adds a column to the view. I don't remember exactly how I got into the bad state, but I learned a few things while troubleshooting it and eventually resolving it.
- The first thing I did according to the instructions in this post was to create the SCL.CFG file and put it program files\microsoft office\office11\forms\1033.
- At some point after this, I started getting the following error when opening embedded messages: “The form required to view this message cannot be displayed. Contact your administrator”.
- I went to Tools | Options | Other | Advanced | Custom Forms | Manage Forms and removed the SCL Extension Form
- From that same dialog, I also cleared the forms cache (“Clear Cache“)
- I restarted Outlook, but the problem kept happening
- I removed SCL.CFG from the directory in step 1 above, cleared the forms cache again and restarted Outlook
The last step resolved the problem. I inquired as to why, and learned that Outlook will automatically go through and re-register every form in that directory.