Error Failed to resize image (original, thumbnail)
I see you use security plugins. This problem comes from lots of restrictions and limitations on your website. Please make sure there is no "/wp-content/ hardening" process in your security plugins or in server.
Something doesn't allow to write/edit/create file in /wp-content/ folder of WordPress. I recommend to go through all settings of your security plugins and disable hard limitation regarding file writing and anything related to wp-content folder.
Could you please leave your forum URL, to allow us to check it.
Yes, this is the link to the forum:
This is the link to the specific thread. The pictures in the thread were added using a smartphone, while a PC would give the error message trying to upload that same picture. From the phone there were no issues to post the same picture:
The picture in this link can only be uploaded from a phone and it causes the same error: