I don’t know what happened - I worked on my website, then somebody called me on the phone. Then I returned to my computer and 2 partials were missing. I looked at them and their data .yml files - they were absolutely fine and correct. This was very strange! After two days of investigation I found that if I rename the file testimonial.yml (a data file for testimonial.html partial) to start with another character - it srarts working! E.g. atestimonial.yml works, while testimonial.yml does not. Of course I renamed links to the data file in the partial as well!
I started the initial empty template as a reference - testimonial.yml and testimonial.html do work there!
I copied all my customized content - .yml, .html, .js and .css - into a new template - and partials that were missing are missing in the new site as well.
Can anybody explain me what may be going on?!