For no obvious reason timesheet gadget shows empty white frame. No configuration changes have been made and in none of the dashboards where this gadget is used data is available.
It is still not clear why it's happening and how to fix that.
One of the causes could be that the plugin got corrupted data on DB, so here are some steps to see if we can solve this problem in the meantime:
If possible, check if there is any dashboard with Timesheet on it and delete it from those dashboards;
Stop JIRA service;
Reboot the DB Server;
Start JIRA Service again;
If its possible, uninstall the plugin (as all data is stored natively within Jira no worklog data will be lost by uninstalling the plugin;
Clear the plugin cache from Jira Home, to better understand how to do this please perform the following:
Install it again
All I can suggest is to try to reenable add-on or uninstall the add-on, remove jira-timeshet*.jar files from your Jira home, see also: https://confluence.atlassian.com/upm/uninstalling-apps-273875709.html, and then install again, or restart Jira or recreate gadget on dashboard, or try to ask Atlassian Support for help, explaining that gadget most likely can not load its configuration for some reason, and probably they can help to figure out the root cause.