Working on a document today and had exactly the same problem; some of the documents refused to undo, the option was greyed out, others working just fine.
Even stuck it in the text editor-no mention of "undo" anywhere...
In the end, I opted for the simple option of closing Scribus and re-opening it. Worked like a charm!
Am I the only one who leaves their programmes open for weeks on end? Interesting to know if they just get "tired" and stop working properly (just like we do), because this is not the first problem that I have resolved this way.