I attempted clearing cache referencing: Clear the Cache
Seemed to generate similar errors.
Removed my /var/lib/openhab2/jsondb/automation_rules.json and associated backup files (nothing I can’t easily re-create fairly easily)
Uninstalled the Add-On, shut down openHAB2, cleared cache, resarted openHAB2, attempted to install the Rule Engine addon and got the following same/similar error repeating multiple times:
2020-06-25 00:33:37.794 [ERROR] [org.openhab.core.automation ] - bundle org.openhab.core.automation:2.5.0 (263)[org.openhab.core.automation.internal.type.ModuleTypeRegistryImpl(357)] : The activate method has thrown an exception
Scratching my head on this one. Installing or Uninstalling this Add-On from the PaperUI generates a slowly turning progress wheel (like expected) however it never stops. Refreshing the page shows it appropriately installed (or not) however nothing in the menu structure on the left still.