hi Nicolas & any one else who might run into this...
I think what happened is this.
I hit the ceiling on my MySQL data base and deleted a pile of old stuff that I no longer need from the Joomla back end.
There was some kind of problem with the Cache in the cPanel and although I deleted a pile of stuff the MySQL data bank did not get smaller...
Enter Siteground step 01:
Question: Deleting files does not reduce the size of my database?
Answer: I have actually found out that there was a cache in the Cpanel and our tool, which was calculating the Database size.
After which I got the error message described above.
Enter Siteground step 02:
Question: 0 The file Cache Storage is not supported on this platform. Why etc?
Answer: I have checked and found that the cache folder was missing. I have created it for your application and now all should be back to normal. Clear the browser cache, re-test the website and if any problem persists provide us the exact steps....
So it sound to me like some one cleared the cache by throwing out a folder and forgot to put an empty one there in its place or similar!
thank you and all the best
Paul