Okay, here's another strange thing I have just discovered.
I've been successful opening a second form I created the same night. The last element I added to the form was a "number" element. When I now open this second form I can no longer edit that number element - even with the previous build version installed! All the other elements function fine. It is very difficult to select the number element, and when I finally do it is not highlighted in grey and the only property shown is the label - which I also can not change!
When I then attempt to open the first file again, I get the same error message as shown below, and I noticed that the "number" element with all its issues is still shown on the right hand box.
I can't attach the files for this issue as they are too large, but I can post them to someone if they want to have a look into them.
Thanks!