There seems to be an issue in OH3 wenn DSL rules have been defined via the GUI: the processing takes sometimes too long and when a rule fires often (e.g. because of an often changing item), it could lead to a Java heap space error and leaves the system completely unusable.
The issue has been discovered a while ago ([OH3] Defining Rules in Main UI increases CPU Load · Issue #1877 · openhab/openhab-core · GitHub), but it seems it’s not happen everywhere.
Workaround: Use ECMA/GUI instead of DSL/GUI