Since gpg4win 3.1.5 is released, I received about 10 error reports concerning pinentry. All report the same problem:
pinentry.exe - System Error The code execution cannot proceed because libassuan-0.dll was not found. Reinstalling the program may fix this problem. The code execution cannot proceed because libgpg-error-0.dll was not found. Reinstalling the program may fix this problem.
Downgrading to 3.1.4 helped in all cases.