This false positive warning is specific to openHAB 2.4.0. If you didn’t apply the suggested logging configuration changes (while upgrading) you can also suppress it by raising the log level in the Console with:
log:set error org.eclipse.jetty.util.thread.ThreadPoolBudget
It doesn’t help with solving Fix for Jetty error when running on host with many cores, which is another issue.
The false positive warning seems to be fixed with the upgrade to Karaf 4.2.2 which uses Jetty 9.4.12.v20180830 and is available in openHAB 2.5.0-SNAPSHOT Build # 1482 (see PR #834).