We've upgraded to 6.8.0 yesterday. We're now seeing 404s on 20-30% of our API calls. The code is all in place, the calls haven't changed but it's 404'ing.
When we try to load from the browser address bar, it returns the same 404. We cannot see anything that's different with the API calls that are failing.
I had a similar issue recently doing a,similar upgrade, 5.8 to in my case 7 beta.
Random api's returning 404 messages.
Initially i thought it may be a permissions issue but changing them manually did not resolve the issue.
I did however find they worked if i changed the api folder!
Can't recall if i simply copied/ dragged them to a new folder or recreated them in the new folder but they worked when i updated the links to them in app connect so i did not investigate further.
Sorry, not a solution as such but you are not alone and this may help towards an actual diagnosis.
Looking at your case, could it be related to the use of _ at start of api names.
"The actions are still accessible from other actions but not from a public Url"
Yes, they're called libraries.
Anyway, after a long unproductive day, we're back in action I think. These days we call 'washing-machine' days. When your washing-machine stops working, you know it's easier to order a new one than pay for help to fix it. Once you've paid, you're no better off, it's not changed your life for the better in any way. It still just washes clothes but you've lost time+money and pissed people off needlessly"