Is your server placed behind reverse proxy ?
Can you check what's written in logs about this error.
One thing i can think of is if SSL certificate is configured for your server, then there might be some mismatch with the chain of certificates. Pls verify that if such is the case.
Have a look at this Article https://confluence.atlassian.com/jirakb/jira-activity-stream-gadget-throws-gadget-common-error-500-973485793.html?_ga=2.129129868.1295211593.1578477449-1758822056.1578477449
Also there is a solved answer in community in this URL pls refer https://community.atlassian.com/t5/Jira-questions/Dashboard-error-gadget-common-error-500/qaq-p/1175383
From the screenshot ( gadget.common.error.500 ), its highly possible that you are facing an issue similar to what is documented here:
You can perform a quick check using curl -v command
Execute the following command from the JIRA server to confirm if Jira is able to communicate with itself:
curl -v <jirabaseurl>
(i) Replace <jirabaseurl> with your complete JIRA base URL.
If the curl -v command does not return successful, please go through the article provided as there a few causes.
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