Y-axis length error during belt test on a very new, previously functional MK3S+  

Y-axis length error during belt test on a very new, previously functional MK3S+

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When I run the belt test, the x-axis works fine. I can hear it "bang" as it hits each side and goes back and forth. But when it does the y-axis, I notice two sounds 1) there's a soft periodic knocking sound for the whole time that the y-axis moves and 2) when it hits the back the motor continues on for 2-3 seconds even when the bed has stopped moving. Then I get the error: Selftest error! Please check: Axis length Axis Y

1. The belt actually is tight, so I am not sure if tightening it further will help. It's inside the Prusa enclosure which makes it a little hard to get to the bottom of the y-axis unless I take it all out.
2. I tried re-lubricating the axis but that didn't do anything
3. When I go into Settings > Move Axis I am able to get it to go from +212 down to 0. But then it goes down further to -4 each time. It's always up to -4 each time. So it's almost like there's some -4 permanent slip somewhere.

I got this pre-assembled by Prusa in February.  I have only used it for 10 or so prints and it worked flawlessly. Then I didn't use it during the summer (3 months) during which it was sitting in a warm garage. And when I tried again I got spaghetti prints, so I tried running calibration and ran into this problem.

Any ideas what might be the issue?
Prusa Orignal 3D MK3S+