LR6 mac Mojave shows a white banner in Develop and Zoom

Hi Community,

with the latest mac update, from Sierra to Mojave, LR6 shows a white banner containing the Image Text like Name, Shuttertime, and others on top of a viewed foto. Formerly this text was transparent displayed over fotos in Develop and Zoom (2nd display). See attached screenshot.

I installed the last available update to LR 6.14 but this behaviour still remains. OS is Mojave 10.14.6 [18G103]. GPU is AMD RX560-AERO-4G as recommended from Apple for older mac pro.

In LR settings / "Leistung" (German)  I disabled the use of graphics processing (unmarked checkbox).

thx, Klaus