Cannot activate custom theme

Hi,

We are using All In One Calendar Plugin version 2.5.37

We have created custom theme as per the guidelines mentioned here: https://time.ly/resources/additional-wordpress-settings/getting-started-with-the-timely-word-press-plugin/wordpress-calendar-settings/

It was working fine before.

However, we by-mistake updated the Plugin to the current version 2.6.7 and our all customization is gone. Fortunatly, we had a backup so we rolled back to previous version 2.5.37 where we had custom theme files.

I follow this path to activate our custom theme
Dashboard -> Events -> Calendar Themes -> scroll down to the custom theme and clicked on “Activate”

I get the message that “New theme activated” – however, I am not able to activate the new theme.

The result is I can only see the basic default theme on front end without any customizations.

Could you please help solving this issue?

Many thanks in advance.

Regards,