Okey, there are some threads about this already, i know..

What the difference here is that in the other threads there is some conclusion about UK phones.

From the begining:
I upgraded to 20.0.012, then experiensed some troubles so I did a soft reset of the phone that solved my problem...

However I cannot install ANY application :(

This is not a UK-phone, that seems to be the consensus in other threads, that this problem only affects theese phones...

So, I have a 5800XM, with 20.0.012 firmware version and product code 0577196(Im in Sweden).

Clock is set to Local time, and correct time and date..

Any ideas?

Maybe a reinstall of the same firmware version? Can't try right now tho, no access to physical windows-computer, only a virtualized one, didnt go so well :)

I kno on one application that had 'expired certificate' error I just set my date back and worked fine. not sure how far back you can go on the 5800 since it's newer. after it installed i put my date to the original, this was on my N95.