I can confirm that with both Manjaro and Arch Linux, tested with kernels 5.3 and 5.4, insync 1.5.7 is not able to be started from the command line.
insync start
insync add_account
results in:
Traceback (most recent call last):
File "isyncd/linux/headless/cli.py", line 108, in ipc_insync
File "socket.py", line 228, in meth
error: [Errno 111] Connection refused
I am strongly of the opinion that since Insync botched its version 3 release (botched is a nice way to put it), the decision to no longer support or develop 1.5.7 headless (CLI) should be reversed. PLEASE set a couple of people to work on getting the headless version in full development again, and reconsider the abandonment of it in the face of a non-working and disastrous version 3.