This sounds like a problem I ran into with my UMC1820 and ADA8200. Yes like Paul said it has to do with syncing.
There is a switch on the back of the ADA8200 right beside the Sync - connector. If you use only one Adat cable running from ADA8200 to UMC1820, then you need to set this switch to: Master 44.1 kHz or Master 48 kHz. Then turn off your UMC1820 and set the push button: OPT I/O to ADAT. Turn UMC1820 on and start up alsamixer and change the UMC1820 to sync to: ADAT. Now it should work again