A model number would be helpful.
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Defrost the unit manually to get rid of the ice buildup.

Then replace the rear cover. You should not have to screw it on.
Plug the unit in and let it run for about an hour.
Make sure that the compressor runs for the full hour. if you do the below check during or shortly after a defrost cycle you may end up with the wrong diagnosis.

Remove the cover and check the evaporator coils. If they have a fairly even coating of frost on them then the sealed system is probably OK.

If the ice ball is where the tubing enters the rear of the freezer make sure that the sealant/putty is OK.

If the coils are OK and the seal is OK I would check the door seals.

Check that they are clean.

Take a 2 inch wide strip of paper, place it between the seal and the unit's frame and then shut the door and pull the paper out to check the seal.
Work your way around both doors every 8 inches or so.

Check the hing side of the seals to be sure they are not grabbing and twisting when the door closes. A light coating of vaseline or white lithium grease usually fixes this.

Note that another low freon symptom is that the compressor will run all the time except when the unit goes through a defrost cycle.