Looks like another TLS issue: first try running curl https://community.openhab.org/tags/c/apps-services/habpanel/widgetgallery.json on your openHAB machine, if you do get the same error either try upgrading your JRE like @Dr_Kuebel did here:
curl https://community.openhab.org/tags/c/apps-services/habpanel/widgetgallery.json
Or follow these steps (including the Java part): [Techy] Getting cURL to work with Let’s Encrypt: unable to get local issuer certificate error – Richy's Random Ramblings
