For those who have or will upgrade to High Sierra, see What High Sierra broke and how you can fix it

The companion article on problems during and after installation is also pretty good.

Unfortunately, there is no fix for ONYX. I miss it. I tried TinkerTool 5. It has most of the maintenance tools I like. However, it is a cumbersome application. ONYX has a nifty AUTOMATION feature which allows the application to run all the customized/pre-selected tools with one click. I cannot locate such a feature in TinkerTool 5.

Also, I miss PATHFINDER, which is not supported for "High Sierra".

So . . . nuke time for me. Will re-install SIERRA and get back ONYX and PATHFINDER. Might give "High Sierra" another try in a few months to see if it will work better on my MBPro (early 2011) . . . and if, in the meantime, I can find a comparable application to replace PATHFINDER.

Not sure if you have tried this
OnyX 3.3.8 for macOS High Sierra 10.13.
It is on the developers website

The developer of Path Finder 7.6.1 says it is compatible with High Sierra. Did you update beyond 7.5?

Thanks for the tips.

However, I did go to the developers' websites for both ONYX and PATHFINDER. I checked for "latest update" - dowloaded and installed, as usual. Then, I re-checked for latest update, and was told I have the latest version of both.

The "newest" version of Onyx crashes and Pathfinder will not even open in "High Sierra." For Pathfinder, the dropdown menu shows at launch, but the options are "greyed out" and the application can only be closed using Force Quit. Too much fiddling about in MacOS 13; no issues whatsoever with either application using MacOS 12.

I haven't entirely given up in OS13, but will wait a while to see if there is a bug fix update before trying again.

P.S. I usually do not update as soon as a new OS is released. This time I went against habit and upgraded too soon.

UPDATE


I erased my hard drive and did a clean install of SIERRA. Problem solved!