Wordpress error: Sorry, you are not allowed to upload this file type

I’m getting the error: Sorry, you are not allowed to upload this file type. When I try to upload a .jpeg file into the Media Library in my WP site.

I’ve tried adding the following code to the wp-config file.
define('ALLOW_UNFILTERED_UPLOADS', true);

I’ve checked the permissions in the wp-content folder and everything is set to 744.

I’ve also tried adding the File Upload Types plugin.

But no love. I’ve been trying/working at the problem for the last couple of days. I’m also getting a timeout error: Error 524 or Service Unavailable while moving around in the WP Dashboard.
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

TIA

I found the problem. It appears that there was a problem with JSON. Solved with a visit to
Settings | Permalinks | Save Changes (without making any changes)

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