Try deactivating all plugins and check to see if this resolves the problem. If it does, activate them one by one – checking the Widget’s section after each re-activation – until you find the which plugin(s) are causing the issue.
For a quick workaround (but not a fix): when on the widgets page, on the upper right corner, select Screen Options and click on “Enable Accessibility Mode”
Thank you for your reply. The Enable Accessibility Mode got me going.