Sorry, this is my forum (with the old version restored): 

http://www.deviolines.com/forum

Oh dear – always hard to pin down this one. And the problem, of course, is that while trying to track it down your forum is, in effect, off-line.

So – main reasons for this are: (1) Something you have installed hijacking the http request and returning data with the upgrade data. We have seen this before with some authentication tools for example. (2) A simple and straightforward JavaScript error causing a conflict. (3) Every now and then we have had reported that simply using a different browser did the trick but there is no proof for this.

Now – this is the difficulty. Both 1 and 2 can usually be deduced by using the browser console tool but for that to happen you need to be actually running the upgrade script. Well – and know how to use the console tool but that can be explained.

Well, I’m not sure to understand (my english is poor, sorry) Now the forum is online.  I tried to upgrade with Safari and with Chrome in a Mac. I deleted the .httacess file, but it didn’t work. May be deactivating all wordpress plugins? 

There are new problems now, with the recent installed plugins: tinymce doesn’t work, private messages either, and there is a strange message when I make a new forum: <!– WP Super Cache is installed but broken. The constant WPCACHEHOME must be set in the file wp-config.php and point at the WP Super Cache plugin directory. –> But many time ago I deleted that plugin. 

The basic forum is running, goodness

if super cache is running at all, you cannot cache the wp page the forum is on – it will break all sorts of things since the page is dynamic in nature…

I deactivated and deleted the plugin wp super caché. Maybe there’s still any configuration related to change.

Solved!

There were two .php files in wp-content folder: wp-cache-config.php and advanced-cache.php. I think they are old files that were disturbing functions of forum. I have deleted them and now I think everything is ok. Uf

If you choose to remove the cache plugin then it does need to be done properly. They do publish detailed information on how to remove. There may still be some additions to your .htaccess file that need removing.

However – there is generally no need to actually stop using a cache plugin. All you need to do is exclude the forum page from the cache routines – it has an exclusion section where you can specify this.

I am surprised, however, that the cache plugin has this sort of effect on the administration side of things bit I am glad that aspect is resolved without too much pain!

I removed it because I had many problems with that plugin and permalinks. I thought all plugin can be removed in the same way, deactivating before, deleting after, in wordpress panel. 

Now I’m using the Cloudflare service, and I’m happy with it.

The plugins are working OK too.

Thanks Mr, Swordfish

Yes – if all works what ore could you want