What could have happened to my NextCloud access

Been running NextCloud on my test machine for a few days now without any problems. Last night before shutting it down, there was a list of OMV updates that I installed in the update manager. Shut the server down for the night and turned it back on this morning. Now I get a 502 Bad Gateway ngnix error when trying to connect to Next Cloud thru duckdns. I didn't touch a single setting on anything other than installing the updates.

Could one of the updates have hosed my NextCloud installation? If not, would anyone have any suggestions on what to look for to try and resolve this because I am clueless, especially since it was working perfectly for three days. I did revert back to the settings for using just local access just to see if it would load and it does so the cloud setup still seems intact. It seems to be something related to the duckdns setup, but I am at a loss since I didn't change any settings.

No, it is running thru docker. I followed the tutorial on youtube by Techno Dad. Should I have done it another way instead?

EDIT: I got it worked out. Basically I just removed everything and reinstalled. Thankfully it was just a test machine and I didn't have anything important on NextCloud yet that I worried about losing. Going to keep running the test server for a few more days and make sure it doesn't happen again. Never did discover what the cause was.

New NextCloud problem regarding security. Now it says 'The "Strict-Transport-Security" HTTP header is not set to at least "15552000" seconds.' I google'd it, but nothing had any real clear instructions for a newbie like me. I need to edit a file, but I don't know which one or where it's located. Any help would be great.

Thank you. Will do this right now.

EDIT: Worked perfectly. Thanks for the help...