Try this in this sequence.
Every file/tree you have a unique preferences. Do you have more than one tree?
In FTMM2, turn SYNC off in each tree.
Sign-off of Ancestry.com in FTMM2, then quit FTMM2.
From your browser (what browser are you using when you go directly to Ancestry.com?, use Fire Fox, more stable than Safari). Go to Ancestry.com and sign off of your account.
Empty the cache of the browser.
Quit the browser.
At this stage, I would prepare to do a permissions repair on my machine. Do you know how to do this? If not, go to the Apple site and search "disk utility". If you're not familiar with this, now is the time to learn, you'll need to use it on rare occasion. This is not difficult nor time consuming, 1/2 hours tops reading and performing, next time you need to use it, 5 minutes.
You'll be restarting your machine after the permissions repair.
Fire up your browser, go to Ancestry.com and sign-on to your account.
Fire-up FTMM2 and sign-on to Ancestry.com.
Hopefully this will right the wrong.
Choosing to keep sync enabled is your personal choice. If you choose to keep it off, and only manually sync, it won't affect you ability to use the built-in web browser and the Ancestry site from FTMM2, this is something I don't use, too many wonkies, it's clunky and slow. I prefer to search direct from the site, I work with 10 tabs open to different areas of the site at a time. You can't do that on FTMM2.
I highly recommend creating and keeping a small but heavy on the media test tree to play around with. You can learn function without worrying about destroying your data.