If the Manual Entry on the Tablet didnt work and if it is happening with only a particular Merchant there could be one of the reason
1. The Payment Gateway Credentials has some issues. It may have some bad data in credentials sitting there . If it just a password issue the gateway respond back with a proper error message as invalid creds. If there is really a bad data i don't think you always get a good error message from gateway. Also make sure the merhcant / user didnt try to update anything on the payment settings on the tablet.
2. Tablet is not able to communicate to gateway due to a internet or other communication issue. If there is no internet you see typically see a 404. There are chances that it went out and didn't get a response or it broke the communication