Error message "Audio dropouts" - recording stopped

Hi out there,

just a short question: while recording stereo mixes from archive files (DAT) via SPDIF thru my UA Apollo whenever I change/insert a plug DURING RECORDING in the - let#s say - stereo mix buss or any other channel (NOT THE RECORDED ONE!) I get the “Audio dropout” message and Cubase stops recording and enters play mode. Now it might be strange to insert plus during recording on other channels, so my question is: did anyone experience this too? Is it “normal”?

Cubase 8.5.20
Mac OS X Yosemite, 16GB