patrike - I'm see this error quite frequently.
The web interface tells me both the CPU and GPU miners are offline, however if I remote desktop into the device I see both are working fine and reporting hashrates?
for the GPU the web says Interface offline
The Claymore Ethereum miner is sometimes not responding to API calls fast enough or not at all, and will for that reason show up as "Interface Offline" in Awesome Miner.
I've had a few similar reports from other users about this, but the problem is reproducable even with the Claymore remote monitoring tool. This indicates that the problem is not specific to Awesome Miner, but in the Claymore miner itself.
You could try to increase the API timeout in the Awesome Miner Options dialog, Advanced section "Miner API connection timeout".