I have an older commercial upright Hobart freezer.

In most cases, an evap that freezes up is either an air flow problem or no defrost cycle. Older Hobarts have mechanical defrost time clocks. It could be locked up i.e., not running so no defrost cycle.
It could be coming out of defrost too early because the sensor that monitors the evap temp is not working properly.
The door could not be closing. If left cracked open, it will freeze up every time as unit never hits temp., keeps unit running. Warm air is seeping in and the moisture freezes on evap.
Lastly, the defrost heater(s) may not be working. Burned out, shorted, broken wire, etc.

Good luck and hope this helps.