@TonyRietwyk said in Qt 5.15.5 cannot load custom designer plugin: Does the installed Designer need 64-bit dll's? Designer or Creator? (Qt Creator has an embedded version of Qt Designer) Recent releases of Qt Creator are 64-bit. You can check by clicking Help > About Qt Creator... I'm not sure about the bitness of the stand-alone Qt Designer . API-MS-WIN-CORE-SYNCH-L1-2-0.DLL seems to be causing a cyclic dependency with KERNEL32.dll. The API-MS-*.dll are usually false positives. Ignore them. I have Qt5.15.5/5.15.5/msvc2019/bin in my system path. This often causes problems... I highly recommend you don't put Qt in your PATH. You can break other apps on your PC that depend on a different version of Qt. @TonyRietwyk said in Qt 5.15.5 cannot load custom designer plugin: Turns out the problem was fixed by copying zlib1.dll into the 5.15.5\msvc2019\bin folder That should go into the folder which contains your plugin, not into your Qt folder. Again, I recommend you remove Qt from your PATH.

Please have a look at the bug report I just submitted on this issue: https://bugreports.qt.io/browse/QTBUG-61059

@Ni.Sumi It's no hassle at all. I've corrected a grammar mistake in the post, but the meaning should remain the same. :) Kind regards.