Although WinMerge v2.16.0 x64 has already been installed on my PC, an exiting v2.14.0 is found and an update is recommended.
I just search the registry for abandon entries and found out that an old WinMerge v2.14.0 entry still existed,
pointing to the non-existing directory path C:\Program Files (x86)\WinMerge. After I've deleted this entry,
PatchMyPC worked as expected again.
[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\WinMerge_is1]
"InstallLocation"="C:\\Program Files (x86)\\WinMerge\\"
"DisplayName"="WinMerge 2.14.0"
Thanks for the update.