Unexpected Error - Now what do I do?

Permalink
This system is (was) running on Concrete 5.7.1 and all was going on quite nicely until it started to give some errors while using file manager, for example when uploading a file, giving an unexpected error message, although the file uploaded OK.

Now things have got worse and trying to get on to the site gives:-

An unexpected error occurred.
An error occurred while processing this request.

Whatever I try I can't move on. I can't find anything that caused this. I created a new installation, imported the database and at that stage got an error about something not redirecting properly and it seeming to want to get to index.php/login despite me not asking for that. It may be a clue but it's not surprising that it didn't work since the various files had not yet been updated.

There's nothing strange going on with htaccess either.

I put in place the various files and changes that made the system and now I'm just getting that message saying that there's been an error.

There's nothing in the web error log and the access log shows the following:-
"GET / HTTP/1.1" 200 1247 "http://c573.mydomain.co.uk/" 
"GET /index.php/tools/required/i18n_js HTTP/1.1" 200 8549 "http://c573.mydomain.co.uk/" 
"GET / HTTP/1.1" 200 1247 "http://c573.mydomain.co.uk/" 
"GET /concrete/themes/concrete/main.css HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/css/font-awesome.css HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/jquery.js HTTP/1.1" 304 212 "http://c573.mydomain.co.uk/" 
"GET /concrete/css/app.css HTTP/1.1" 304 212 "http://c573.mydomain.co.uk/" 
"GET /concrete/css/jquery-ui.css HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/events.js HTTP/1.1" 304 210 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/underscore.js HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/picturefill.js HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/bootstrap/dropdown.js HTTP/1.1" 304 210 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/bootstrap/tooltip.js HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/jquery-ui.js HTTP/1.1" 304 212 "http://c573.mydomain.co.uk/" 
"GET /concrete/js/legacy.js HTTP/1.1" 304 211 "http://c573.mydomain.co.uk/"


All responses were 200 or 304, though I suspect that the next message might have been more helpful! Maybe it would reveal what it was trying to find/get.

How on earth do I start to untangle this mess?

It's also the samne using different computers, browsers and operating systems.

Had I been asked what to do I'd have said restore from backup but the backup was taken after the problem occurred and so there is nothing useful available. Clearly I'll be looking at our provider's backup strategy, but that's another story.