Hello Oleg,
A while back you and I traded some private messages on the timestamp and lastmod topics of the generator script. Your advice/insight was that it was probably a server issue whereby the server may not be providing the correct http header (Last-modification), or not including it at all in the server's response. I forwarded your info to my hosting support and after they looked at some things, they replied to me with the following...
***********************
I have thoroughly reviewed the provided information and was able to re-create the explained problem. Indeed this is something pretty interesting and I have a theory that since there are two web service on the server (Apache and xVarnish) and the fact that you have both PHP and HTML code within the file causes some serious mix up with both of them and when the handler is not active, while the static content is being processed within the file, the dynamic pages are not, since they are defined within an HTML file and the lsphp handler is not enabled.
I think I may have found a solution you can use and remove the handler. What I did is I simply removed it and renamed the HTML file to a PHP file and despite the handler missing from the .htaccess - every page was generated as it should be, as seen here:
> [ External links are visible to forum administrators only ]
That way, since the file is PHP, it will get processed by Apache. Unfortunately though, after this change, I am not able to see the last modified header using this tool:
> [ External links are visible to forum administrators only ]
since I believe that there needs to be a PHP function that actually shows it but I may be wrong - I am not really a developer myself and right now I would simply be assuming which is highly unprofessional on my part so I will leave this information here and hopefully you may get an idea of what needs to be done. Every change I made for testing purposes were reverted back to their original state.
I am not sure if this is something you would like to do but this is really the only thing I can think of at the moment, at least considering the current server setup with xVarnish. If the application's developer has a better idea and needs something enabled or done on the server in order for this to work, we will be happy to assist you as much as possible.
Please let us know if you need further assistance.
***************************************
Can you let me know if the info provided by the hosting support seems reasonable, or makes sense? Or can you tell me what I should ask them in response to what they have said to me? Thank you in advance for your help.