I have resolved the issue with this league as follows:
Turned on Commissioner Mode. Tried to shop players; menu option was still not available. Drag and drop (shopping) still did not work.
Changed 'Act as' to another team. Tired to shop THEIR players ... that worked fine, the menus were back to normal.
Changed 'Act as' back to my team, and turned Commissioner Mode off. The menus (and player shopping) now work normally again.
To me, this is perhaps a small bug somewhere, but since I have resolved it, I'm not terribly worried about it unless someone wants to follow up. Thank you all for your good suggestions, it got me to thinking as to different ways to test and solve.