I am still experiencing this in 3.2.1.
Users with Universal Audio recording interfaces (e.g. the Apollo rack units) will all have the plug-ins installed on their Macs, but they may take their Mac laptops away from the studio for a while, perhaps to work on something in Audacity that doesn’t require these plug-ins.
Universal Audio interfaces are the hardware DSP powering these plug-ins, but the plug-ins themselves might be checking to see if the UA unit is plugged in at the time of testing / activation. Additionally, the plugins themselves are licensed internally on the UA hardware—there is no iLok HASP or Aladdin dongle, it’s all done over the internet and the license is updated inside the interface’s memory—so the plugins may also be checking for activation keys even though the UA isn’t connected.
Audacity 3.2.1 is hanging indefinitely at checking one such plugin, and since there is no ‘skip’ button I must either Stop or Cancel.
I came back this morning after leaving it overnight. 12 hours. No progress.
I pressed “Stop” and will be marking all of the UAD plug-ins as “Disabled” for now.
Here’s a screenshot of what that list looks like in Audacity. Note the pathnames start with !UAD/aufx/xxAU, which is not a real path. The plug-ins in the “VST” folder are references to the real ones in the UAD collection.
After marking the UAD plugins as “Disabled,” I relaunched Audacity. Now there is a second Audacity icon bouncing in my dock. The plug-in validator appeared and was still stuck on one of the UAD plugins. Eventually the icon stopped bouncing.
I would suggest that Audacity break out the plug-in validator into a totally separate executable, the way that Logic Pro does, and monitor this child process for responsiveness. If it takes more than 10 seconds for the child process to validate a plugin, kill it and note the condition so the end user can decide how to handle it later.
Real-time plugins are not available in Audacity until this scan completes once..
Without completing the scan, the “Add Effect” drop-down menu is empty except for “Get More Effects…”
I’m attaching the crash log caught by macOS when killing the spawned validator process, and also a screenshot of what the UAD plugins look like in Audacity.

audacity-plugin-validator-hang.txt (22.4 KB)