First please delete your cookies, then click on the "Mark All Read" ink located in the forum footer. If it doesn't help, then this is not wpForo issue. Try to login with put account and check it, it maybe a browser or device specific issue.

You can fix Error 502 by doing the following methods:

1. Refresh your browser and reopen it: First, you need to close your current browser and hold up a couple of moments and reopen it. It is because Sometimes your browser may stop responding due to accessing the heavy traffic of data at the same time or maybe an error page is inserted in between the pages which are loading. Even you try on another browser as well.
2. Reset Internet configuration: You need to restart your internet modem. to do this, Unplug your internet modem and hold up for 5 seconds, after it completely turns off, Plugs it again and Wait for your computer to establish a connection again.
3. Disable CDN temporarily: The Content Delivery Network or commonly known as CDN improves the loading speed of a website but it can generate error 502 as well due to the firewalls. To overcome this issue, you can disable the CDN temporarily.

I have attached the screenshot below.