I just found another thread in the other locus forum, which has much more information and discussion:
https://help.locusmap.eu/topic/34422-cache-aktualisieren-funktioniert-nicht-mehr-locus-map-classic-3-70-7
https://help.locusmap.eu/topic/34422-cache-aktualisieren-funktioniert-nicht-mehr-locus-map-classic-3-70-7