Tested a little more, still happens, but not if Opera is already running in the background. This time it looked like the button was visible a short moment then I started Opera but then disappeared.
Just to let you know that the last 3 days of turning on the tablet the problem has now gone,
so I guess there was something amiss with one of the 2 games that I have now removed and would
like to thank sgunhouse for pointing me in the right direction.
Without more information, we really can't say. Does your device have enough storage for the file? Does you network block certain types of files? (What types have you tried to download?)
One clue - how long after you start the download does the error message appear?