@thelonelycoder ,
@RMerlin
This is on my main router, the RT-AC86U.
Well, that was fun: I did try reinstalling my amtm scripts once again, startring with Entware, on a alternative simple USB flash drive. That also didn't work under 386.7 (beta1), as I still see a fast scroll off the screen to the effect of "Address is invalid....xxx........etc...", then "Entware (aarch64) install failed" when the amtm menu comes back.
So I decided to do a full factory reset this morning under 386.7, and manual input of all settings (no settings were restored from backup files): full wipe of jffs, full wipe and reformatting of the attached (main, SSD based) USB storage. I started fresh in amtm, "fd" utility to ext4 with journaling (one partition), and on to creating a (5GB) swap file after one more reboot cycle. So far so good, but no, not really: this still fails to install Entware from within amtm with exactly the same error as before.
OK, lets try a downgrade to 386.5_2: immediate success, no problems at all installing any amtm utilities. I still can't believe I am the only one seeing this!? I'm starting to wonder if nearly everyone else has moved on to newer (and less problematic) routers. I guess I will have to do as
@L&LD suggested, wait this one out until we see a full final release of 367.7 and then wait just a bit more to see if anyone else runs into this problem.