Carlos E. R. wrote:
He types the password, selects it, then does "control-C" to store the password in the clipboard, thus in memory.
Then he tries to install a second rpm the same way, and hits "control-V" to paste the password on the dialog. On three computers this works, on a fourth the password is gone from the clipboard.
The above is correct. Luckily, I finally managed to solve that riddle. I decided to re-install the plasma5-workspace and the dependent libraries. After that procedure, everything works in the same way as my three other Linux machines. Somehow something necessary for keeping the info in the clipboard must have been missing from my first installation. You absolutely put me on the right track. Thanks! Per Inge Oestmoen