No users, not even the admin can upload attachments anymore. They see a blank page instead. Meanwhile, the filenames appear in the WordPress Media library, but they got no content.
Using wpForo version 1.3.1 and wpForo Embeds 1.0.4.
wpForo Embeds is Youtube embedding addon, it doesn't have any relation to attachment uploading. Please read this doc and explain more how you use the attachments:
https://wpforo.com/docs/root/topics-posts/atatch-file-to-post/
The problem is the latest version of WP Smush. Looks like it's not compatible with wpForo.
I'm sorry but I have no idea. We have a lot to do before we can start working on hundreds of thousands wordpress plugin compatibility. You should use whether... or ask the WP Smush to help you. We use standard WordPress functions, we don't have extra things here.