Well, if I had to solve this, that is, if I had a customer wanting to pay me for solving this problems (very low probability :-)) I’d first check if the client has been well and truly compiled against the same version as the server, and if yes, I’d install same Debian version in KVM, install snap and snap lxd, check that if works, if not whine on LXD github issues, if yes try to compare behaviour of snap (working) version and compiled (not working) version, while also browsing at the LXD sources to try to understand, searching for similar issues on the internet, until the reason becomes clear or blood spill from my eyes.
If you want to live with source, you have to realize you can if not actually die, have at least, painful experiences with it.