We tested on a similar rig (RX570 + RX5700, Windows 10 1709, AMD driver 19.9.1) and we didn't encounter any problems. Does PhoenixMiner 4.5c work on the same rig? What is the version of the drivers? Also, please try to run PhoenixMiner 4.6b only with -list (i.e. Phoenixminer.exe -list) and see if it shows the list of GPUs properly. A possible but unlikely reason - Windows defender may be blocking or even deleting PhoenixMiner.exe.
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Edit3: Turns out Windows Defender was blocking the miner... Haven't seen it block something like that before....
Thank you for finding the cause of the problem on your rig. We were scratching our heads because even with latest Windows updates we couldn't reproduce the problem. Frankly, the behviour of Windows Defender is becoming really idiotic. We are already disabled it permanently on all our test rigs (and even on developer PCs) from the start but we will stop short from recommending this for everyone. Just keep in mind that whenever you see strange behavior or unexplained errors, just try with disabled Windows Defender to be sure that it is not causing problems.