[Elecraft] KPA500 LCD-Display mysterious failure

[Oli, DL1OLI]

Dear Elecraft-list, 

today I powered my KPA500 #1584 up and got no display on the KPA500. The backlight is working properly but the LCD shows nothing.

The Amp is working OK, it communicates with the KPA500 Remote utility, it communicates with the K3 and does 500 W Output as before. Only the LCD shows nothing. 
Even the keys for Menu and the bandkeys are working fine.

After some time, after I removed it from power complete several times, opening the top Cover, controlling remote, flashing new firmware ect. the LCD is mysteriously working again. 
Any tips to this? What to check for? Thank you.

vy 73 Oli
DL1OLI/AJ4UR