Library thumbnails disappear after upgrade

Hi Folks

After upgrading from 2.8.5 to 2.8.6 I appear to have lost connection to my thumbnails in the image library.
In fact from wherever I try to access my thumbnails, I get the attached screenshot.
I’ve done nothing but upgrade using the UpgradeMODX Extra.
I have checked Media Sources paths.
I’ve looked at System Settings for phpthumbof but everything seems to be in order.
There are no messages in the Error Log.

Any pointers would be appreciated

Many thanks

If you are using PHP 7.4, take a look at this post: