Hi
This may help.
Step 1 empty your cache and try again.
Safari: Preferences: Manage Website Data: find the site in question and delete.
Also
Last week Safari 14.1 there was a bug in the version last week that caused problems with sites that use JavaScript. This included banks, ebay and others. I have posted a summary on the recent situation that caused page load issues including tests to identify the problem version of Safari, workarounds and an updated solution including how to find the update download ( as of May 4). NOTE: Scroll /read to the end for a recent update and summary, first solution was a workaround.
https://discussions.apple.com/thread/252710814?answerId=255146246022#255146246022
I hope this helps