Based on your documentation, where you show the problem and the solution, it looks like you got the error for illegally specifying the old style of "Window on a field" when you wanted to specify the new style.
It used to be that C 20/25 would create a C 25 field that fit in 20 characters of space. However, back in the 4.1 release notes somewhere there's a mention that they changed it to 20/C 25 to reduce confusion. You'll want to always specify the visible size of the control first, then the slash, then the field type, and finally, the data size of the control.
There is an option that allows things to still work the old way. I'm not sure but I think its Option 45.
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