Summary: Do not upgrade to v21.0.5 using Zero Downtime Upgrade without first contacting Broadcom Support.
Update 2023.03.02 18:20 CET: Broadcom has acknowledged that this problem is due to a bug in AE v21.0.5.
I have not seen an announcement about this so I thought I would share our experience in the hopes others may be spared needless trouble.
Eleven days ago, Broadcom released AWA 21.0.5. We had been waiting for bug fixes delivered in this specific release, so we were understandably eager to install the new version. Unfortunately, we ran into a problem when using the Zero Downtime Upgrade feature to upgrade one of our systems from 12.3.9 HF2 to 21.0.5.
Everything went OK until step 4. Connections in the ZDU Wizard. During that step, we clicked Disconnect All Agents and then Disconnect All Users. This closed connections to the old-version Communication Processes, and triggered the switch of the PWP to the new version. As soon as the switch happened, the problem manifested. Normal (non-Java) Work Processes began to die with segmentation faults, writing UC4Dump files and forced traces.
Some WPs would stay up for a few minutes, but others would die almost immediately after starting. Restarting the WPs worked fine, but they would soon die again. Eventually, more WPs were down than up. Also, the WP traces were beginning to fill up the file system, so we chose to stop the system and call Broadcom Support for help.
Broadcom Support is currently investigating the problem. Their preliminary guess at a root cause is a known bug in 21.0.5. The bug is apparently related to ZDU, and a hotfix is expected soon.
If you intend to upgrade to v21.0.5 using Zero Downtime Upgrade, I strongly recommend that you contact Broadcom Support for advice first.
One more thing: I believe Broadcom should take a more proactive stance in notifying customers of known serious problems affecting releases that are publicly available. The Release Notes tool on the Automic Downloads site lists no known problems for Automation Engine 21.0.5. I searched for relevant KB articles and found only one partial hit: KB article 258694. (That bug is supposedly PostgreSQL-specific though, and we use Oracle; also, the symptoms described in the article are somewhat different from the ones we experienced.) We had been waiting for many months for bug fixes included in v21.0.5, and had reported several of these bugs; so Broadcom Support knew or should have known that we were intending to upgrade to this version. AE v12.0.5 was released almost two weeks ago, but Broadcom informed us that they knew about this problem only after we had run into the problem and opened a support case.