We almost have the same issue i.e sandbox VA is inactive/unresponsive since 7th Jan and all aggregations keep failing since then. The VA is stuck in ‘VA upgrade in progress’ state. I had to reboot the CCG service and the VA/cluster a few times but still no luck.
I raised a case with support team and they are looking into it.
As per their advice, I had to check to make sure SSL inspection is disabled for all outbound traffic on the sandbox VA and also the required primary & region-specific AWS URLs are whitelisted, but still without any luck.
I would recommend, raise a support case, if possible run the STUNT script diagnosis on the VA and send them the logs to have a look.
Depending on the size of the legacy system(s) footprint within the enterprise, you may want to look into having a TLS version translation layer (e.g. to terminate TLS 1.3 on a load balancer or proxy [ NGINX or HAProxy for example], and re-encrypt with TLS 1.0 to reach the legacy system).