WL newbies needs tutorial on basic album-mastering workflow

Kapverden, the following seems to work for recording only the external gear (which if I’m understanding correctly is what you want to do: Playback the unmastered file in Wavelab, out thru the external gear, and record only that external gear back into Wavelab). The following method allows you to do that and audition either just the external gear, or the external gear plus the plugins that follow it (which are not getting recorded).

In the record dialog:

INPUT: AUDIO INPUT (HARDWARE)
SET INPUT: RECORDING CHAN 17+18. (the inputs set on your EXT gear plugin).
If you UNCHECK THE MONITOR BUTTON, that will give you monitor of the whole chain, including the eq and comp plugs in the master section.
If you CHECK THE MONITOR BUTTON, that will give you monitor of only the external gear.

But regardless of the monitor button status, only the external gear will get recorded.

You can toggle the monitor button during recording and it won’t effect the recording. Only the external gear will be recorded.

I got your playback error message once when setting this up, but I can’t replicate it. The only sure way I can replicate it is if the record file format is set to a different sampling rate than the file playing back, which is not what happened to you.