Beko Fridge Freezer – Ice sheet at bottom of freezer

I have a Beko CJFF1582W frost free fridge freezer, about 3.5 years old. A couple of days ago I noticed a sheet of ice on the bottom of the freezer (not something I’ve ever seen before). I removed it, but it has reappeared again a couple of days later.

I assume the freezer has a gully to collect water (in much the same way as the fridge)? If so, I can see water running down from this, so it is presumably overflowing due to a blocked drain hole?

Unfortunately, it’s covered by a back panel similar to the one in this video:

https://www.youtube.com/watch?v=I5ZUc6P2rBI

and after unscrewing it, I find it simply won’t budge.

Is the blockage most likely to be ice? If so I assume a full lengthy defrost is my only option? Or is there a quicker alternative?

One further question. When I pulled out the fridge freezer to investigate this issue I found the following on the floor behind it, so it must have dropped off from somewhere (though it’s not obvious to me where exactly):

https://imgur.com/a/Y27xAyN

It’s a fairly inflexible rubber like strip (about 15cm x 5cm) covered in a very sticky tar like substance. Any ideas where it has come from and whether it’s loss is important?

Thanks