With ABRP App V4.2.4 the error is solved. Now it shown a no speedlimit sign instead of 999 km/h.
It would be better to show no sign at all if the street has no speed limit at all.
Same here: ABRP App V4.2.3 with CarPlay and iPhone SE (2020) iOS 15.4.1
After update to V4.2.4 now the problem was fixed - the traffic sign for unlimited speed is shown.