Bitcoin Core v0.21.1 has no problem for solo miner. mining cmd: getblocktemplate and submitblock got no changed and work well.
cgminer 4.11.1 need to fix it to work on gbt_solo mode. The cgminer got three work modes: has_stratum, has_gbt and gbt_solo.
I found it is very hard to run c code when segment faults often pump out. You have to work out yourself as solo users.
And by the way, I am just using my desktop computer for lotto solo mining. No other dedicated device!
The latest error message is: No servers could be used. Exiting.
(oh my goodness! I am still solo mining. Do i still need a server! ?)
I read from a reddit post that Virtualbox can be installed to create a stratum server. Did you use Virtualbox?