Undo option is not activated OOTB. Go to [1] and check the details to activate it. Once activated, you can undo any changes. [1] https://experienceleague.adobe.com/docs/experience-manager-65/administering/operations/config-undo.html?lang=en#default-configuration 

Can you try to stop the bundle/service named "com.aem.demo.core.models.HelloWorldModel" and try again? It seems like that service is not correctly instantiated

Checking the error[1] , it seems like an issue with the S3 config. Do you see the same issue when replication is triggered manually? Also, setup a DEBUG logger on "com.day.cq.replication" to see if the issue is related to the replication bundle. Lastly, as others mentioned, try adding the service us...

Yes, you can achieve that by Add the multivalue properties cssProcessor and jsProcessor on the clientlibrary node. See [1] for more details [1] https://experienceleague.adobe.com/docs/experience-manager-65/developing/introduction/clientlibs.html?lang=en#usage 

Setup a DEBUG level logger on the following classes: org.apache.jackrabbit.oak.security.authentication.ldaporg.apache.jackrabbit.oak.spi.security.authentication.external Once done, try to access AEM via LDAP and notice what you get in the logs.

AEM 6.1 has been out of support for a long time. 6.2 and 6.3 have also been out of support. So, I would recommend upgrading as soon as possible. That said, for recovering a username's password, you need to login via admin and go to /useradmin to reset the password

You can find detailed docs related to headless AEM and templates at [1] [1] https://experienceleague.adobe.com/docs/experience-manager-learn/getting-started-with-aem-headless/content-services/chapter-4.html?lang=en#understanding-the-role-of-templates-in-aem-content-services 

Please log a ticket with Brightcove support. If they find anything wrong with core AEM functionality(Which I highly doubt), update here and we can further debug the issue.

You could try manually overlaying the StyleTextEditor.js file under /apps. More details at [1] [1] https://experienceleaguecommunities.adobe.com/t5/adobe-experience-manager/configuring-rte-plugin-for-content-fragment-editor/qaq-p/230868 

Check if you have allowed the host making the POST call in the "Apache Sling Referrer Filter" If still an issue, check the log files on AEM side when you make a request from the dispatcher to see what's going on. Also, might be a good idea to change dispatcher log level to DEBUG and see if there are...