When dealing with externals, or abstractions, max needs to know where the source file is. By default max looks in the Cycling74 folder, the Patches folder, and the examples folder. You can either put your externals in any of these folders, or you can tell max to look in another folder of your choosing. You can do this by going to Options > File Preferences and adding the folder you want.
I know that of course. That’s why all my other externals work fine.
I think I found the solution – I had two versions in my patches folder 1.16 an 1.17. Maybe this was confusing for MAX since both versions have the same name. After I deleted all the stuff and unpacked just what I need again, everything works fine now.