I am having similar issues on an Ubuntu 18.04.5 virtual server - It starts, I can configure user and devices, but after some time the server stops responding, so I restart it. Then I get in "Your credentials are not valid." when I try to log back in using the same account.
I re-installed to get this to recover for a bit, but it returned.
Linux linode 5.10.13-x86_64-linode141 #1 SMP Thu Feb 4 13:56:42 EST 2021 x86_64 x86_64 x86_64 GNU/Linux
errors from "-debug -v"
# ./c2-3.1.1_amd64_linux -hostname xx.xxx.xxx -https -debug -v
info | 2021-02-23 22:21:53 **********