It sounds like you're dealing with a frustrating issue where workflows on a specific SharePoint site aren’t functioning, even though other sites with workflows are working fine. This is often caused by a few potential factors.

First, it's good that you checked the storage on your tenant, as that could have been a reason, but since you're only using about 4% of the available space, it’s likely not the issue. Here are a few things you might want to look into:

Workflow Throttling: Sometimes, Office 365 can throttle workflows if too many are running at the same time, causing delays or failures. You might want to check if there is an overload of workflows happening on that site.

Permissions Issue: Ensure that the user account running the workflows has the proper permissions on the site and list. Even if other workflows work on different sites, there could be unique permission settings on this specific one.

Corruption in the Workflow: It’s possible that something got corrupted in the workflow itself. You can try republishing the workflows from SharePoint Designer to see if that resolves the issue.

Workflow Service: Check if the Workflow Manager service is running correctly for that specific site. Sometimes, restarting the workflow service or resetting the site’s connection to it can help.

Timeouts: Since you mentioned long delays in loading the workflow page, there could be a timeout issue. Increasing the timeout values in the settings (if possible) might help or check if the site is experiencing performance issues.

It might be worth trying a new workflow on that site as a test to see if it runs, or migrating one of the broken workflows to another site to check if it’s the site itself that’s causing the issue.

At NGS Solution, we specialize in troubleshooting SharePoint workflows and can help you diagnose and fix issues like these. If you’d like expert assistance, feel free to reach out. Have you tried republishing the workflow yet? Let us know, and we’d be happy to help!