This seems to be similar to issues others are having after the Safari 14.1 update 4/27/2021 that was part of the Catalina 10.15.7 update and it relates to JavaScript.
If you have recently updated to Safari 14.1 (Menu: Safari : About ) ( note the update may have been automatic ) test the following to help you isolate:
Turn off “Enable Javascript” in Safari: Preferences: Security
See if this resolves the the issue loading pages
This is not a solution since pages are only partially complete and lacks any Javascript functionality.
I have been working on this issue and so far the only current workarounds / solutions are:
1) Use Firefox ( My preferred solution while I wait for Apple to release a fix/update. )
2) Use Chrome ( I avoid due to security concerns and data harvesting by the big G. )
3) Install the Apple Developer preview of Safari 14.2 ( https://developer.apple.com/safari/technology-preview/ ) NOTE this is a preview that coexists with the current version of Safari ). Since it is not an official release there are risks. I have tested and it works perfectly opening problem sites.
4) Apple will tell you to reinstall the OS BUT I have not tested and is time consuming...Some people have posted that a reinstall does not resolve the issue.
I am still searching for a fix.
Bottom Line: At this point this is most likely an Apple bug/issue related to 14.1 and they need to offer a fix other than an OSX reinstall and that reminds me of the #1 Microsoft Windows issue solution!