Got this error (see image) as I was about to create a new list item.
I remembered that this list had a custom field applied to it as a column before. After some scouring around, I realised that the field xml file (fldtype_.xml) has been removed from <12 Hive>\Template\XML folder.
Remove the column from the list via-Settings –> List Settings. Note: Please be aware that this was applicable to my solution as we did not require the custom field type anymore.
If however the field type is a requirement, then a re-deploy of the solution package (assuming that it has been packaged up in one!) would place the xml file back into the appropriate location. And after an iisreset all should be back to normal.