It certainly sounds like the -1 memory_limit issue noted in this thread.
If the problem just happened suddenly without you changing anything yourself, then it is very likely that something changed on your web server.
Please check in with your web host and ask them if they have changed anything on your hosting account recently and, if so, what.
Also, please confirm what your PHP memory_limit is currently set to.
Better still, if you could post the link to a phpinfo() file so that I can take a look at your PHP settings, that your would great, thanks.
Create a new file in a plain text editor with the following code: <?php phpinfo(); ?>
Save the file with a .php file extension (for example, 'phpinfo.php').
Upload the file to your web server.
Copy the URL to the file in a post below..
When implementing the workaround I posted in the other thread (link above), please make sure that lines 25 to 27 inclusive of the 'admin/classes/thumbnail.php' file are exactly as follows:
$memoryLimit = (ini_get('memory_limit') == '')
Hopefully my notes above will point us in the right direction and be a step closer to solving your problem.
If the problem persists, I would be more than happy to investigate further but it would help if I had access to your web server. If this proves to be necessary and you are agreeable, then I'll send you an email address where you can forward to me your FTP login credentials. Thank you.