When it's so many rows it sounds like it is a board issue.
If you go to the menu Test->Switch Edges, with all the four cables disconnected from the bottom of the CPU board (J206/J207, J208/J209, J205 and J212.). Then no switch shall be affected. If J205 and J212 are connected, only switch 24 shall be closed in the switch matrix (See picture below how the matrix shall look like, with a square only on position 24.), if you have one or more whole rows/colums closed it's a problem on the CPU-board. U20 (and sometimes also U14) when it's columns. U18 if it's one or more of rows 1 to 4, and U19 row 5 to 8.
Switch 24 closed (resized).png

Yes, it sounds like U20. It's not always you see the problem in the displayed switch matrix in Test->Switch edges.
You can also test the CPU-board when you disconnect all the cables at the bottom of the CPU-board (J206/J207, J208/J209, J205 and J212.). Go in to Tests->Switch Edges, and then short-circuit pin 1 on connector J207 to pin 1 on J209 with a cable, and see if you only get switch 11 affected, or the whole row.
If the whole row is affected, then the problem is on the CPU-board, and probably it is a broken U20.