Leaky 1/2” NPT valve with strange notches. Could they be the issue?
Since this is a German made valve perhaps it was designed for sealing with the hemp fiber and paste sealant system used in Europe.
In this video the technique is to cut notches, perhaps to engage the hemp. The pipe in this video is parallel, not tapered. Is the valve threaded tapered or parallel?
EDIT
See https://en.wikipedia.org/wiki/British_Standard_Pipe
Two types of threads are distinguished:

Parallel (straight) threads:
British Standard Pipe Parallel thread
(BSPP; originally also known as British Standard Pipe Fitting
thread/BSPF and British Standard Pipe Mechanical thread/BSPM), which
have a constant diameter; denoted by the letter G.
Taper threads:
British Standard Pipe Taper thread (BSPT), whose diameter increases or
decreases along the length of the thread; denoted by the letter R.

Your valve has the designation &quot;GD1&quot;. Could this be straight threads (non-tapered)? Presumably the hose threads on the output are parallel threads so if the &quot;G&quot; designation does indicate parallel threads, then it may be referring to the hose threads only.