Found the solution 
The problem was in .htaccess
Solution was this line: AddType text/html .html

These days we must remember the important things in life. And these are  not Elxis, PHP, javascript, coding and web sites. The important things are You, Me, everyone of us. I wish you to be healthy, happy and creative. As for the Elxis, we are here to help, this will never stop. Worry about other things, not for Elxis, we have you covered 

Happy new year 2026 to all.

To my friends here,

As we welcome 2026, I would like to extend my warmest New Year wishes to everyone in the Elxis CMS community. ❤️

This forum has always been more than a technical space, it is a place of mutual respect, thoughtful discussion, and genuine willingness to help others grow. That spirit is what makes this community truly special.

In the coming year, I wish you all good health, peace of mind, and safety for you and your families. May your work progress smoothly, your ideas become reality, and your challenges turn into meaningful learning experiences. Thank you to everyone who contributes their time and knowledge, and sincere appreciation to datahell for maintaining this space.

May 2026 be a year of steady progress, quiet confidence, and continued connection. Happy New Year to you all.

I figured this out - my own error. I had mistakenly removed a backend module position on the primary site. i deleted subsite, corrected the error and created the subsite again without issues.