Not sure where to start here. I'm having serious problems with my Neon install that have started in the last hour, seemingly from out of nowhere.

The only trigger I can think may have caused it is a recent update. I ran 'sudo pkcon update' and accepted a handful of updates without paying much attention to them, as I do a couple of times a day usually. I do this when notified by Discover that updates are available because it seems to be quicker than applying them through Discover.

The first problem I noticed was my system had forgotten the file association for an ods file I tried to open in Dolphin. I selected 'open with...' Libre Office, told the system to remember the application association, but to no avail - the document opened in Calc alright but when I tried to open another spreadsheet a minute later the association hadn't been saved.

The next problem was opening up my Zim notebook I got a "permission denied" error message. I got similar error messages attempting to open Firefox, Thunderbird and QuodLibet. All the other applications I've tested so far are working fine, but a few seem to be using unfamiliar (fallback?) icons. I reopened Dolphin, which I have set up to show a terminal in the bottom panel and instead it showed the error "Terminal cannot be shown because Konsole is not installed. Please install it and then reopen the panel." Konsole is very much installed, which I can confirm by accessing it from within Dolphin via "Tools/Open Konsole". I've checked a few other files and all file associations now appear to be forgotten or inaccessible. And finally (for now) the application menu on my panel is there but completely empty.

Any ideas?

Needless to say, I'm desperate. I can usually muddle through most problems that pop up but Google isn't helping in the slightest on this one so any help would be appreciated.

Just logged on to a fresh profile and it appears to be working fine, but obviously not much use. Is my main profile corrupted beyond salvation? Am I going to have to rebuild it from my backup or is there an easy fix?

There was an easy fix! The cache had become corrupted and I'd been locked out.

# sudo rm -r ~/.cache

fixed everything.