I can build projects by hand individually. For the nightly builds, we throw probably 10-15 projects into the queue at a time, and wait for them to filter through. Upon a recent upgrade of both jenkinsci/blueocean and the plugins, these builds now hang indefinitely and no new builds can be started successfully. Restarting Jenkins master fixes the issue.

Connections are done over a Docker cloud, using the TCP connection and the "attach Docker container" option.

The errors on the node hosting the build instances is:

DockerContainerWatchdog Asynchronous Periodic Work thread is still running. Execution aborted.
Oct 13, 2020 4:15:41 PM INFO hudson.model.AsyncPeriodicWork doRunDockerContainerWatchdog Asynchronous Periodic Work thread is still running. Execution aborted.
Oct 13, 2020 4:20:41 PM INFO hudson.model.AsyncPeriodicWork doRunDockerContainerWatchdog Asynchronous Periodic Work thread is still running. Execution aborted.
Oct 13, 2020 4:25:41 PM INFO hudson.model.AsyncPeriodicWork doRunDockerContainerWatchdog Asynchronous Periodic Work thread is still running. Execution aborted.

The console logs for each build on the Jenkins master show:

Started by timer
Obtained Jenkinsfile from git git@gitlab.company.org:ns/repo.git
Running in Durability level: MAX_SURVIVABILITY
[Pipeline] Start of Pipeline
[Pipeline] node
Still waiting to schedule task
'Ubuntu 16.04 Kinetic-0006iph8pkfzg on docker' is offline

 EDIT: Just verified as well that I can overload our build agents and the queue will eventually clear (overloaded by 1 extra build). So maybe it's the number of tasks? I try to push something like 16 builds at the same time, with the bandwidth to handle 4 at a time, and each build takes probably 10 minutes.