I was trying to install Package Manager, and during the initial setup, HPM was stuck on downloading package manifests. At this point, I opened a new browser tab to http://hubitat.local and realized that the HE hub was stuck in a rebooting loop. It would progress through loading devices, loading apps…

Thanks to everyone for your help! My Diagnostic Tool is the latest version, 1.0.97. It turns out to be a failing power adapter (?? My Hubitat is less than one month old...). I changed to my phone's power adapter and the hub was able to soft reset & reboot successfully.