No more uploads & updates possible

Dear developers,

From one moment to the next, no uploads were possible anymore, this concerns uploads, updates and media.
No, it was not file permissions.

I found the solution on the internet that worked:
– I created the folder /wp-content/tmp/
– and added the next line in the wp-config: define( ‘WP_TEMP_DIR’, ABSPATH . ‘wp-content/tmp/’) ;

Now, although my web page works well, of course, this can not be a permanent solution, because something went wrong in wordpress.
I have encountered this problem at various forums.
Where does wordpress store normally temporary files?

I do not know if the following problem is connected to the above:
Some time ago, media files regularly disappeared from the library inexplicable. That means 1 picture of the 10 scaled. The picture was no longer recognized correctly by WP.

I hope you find out where the problem is.
Sincerely,
Rovanov

The page I need help with: [log in to see the link]