Drupal 7.x

Hello everybody, my friend has been running a site for long time, lately they have faced a problem:

Frontend is working fine, backend is accessible but when you click on any link in the admin menu it displays 500 error, in both frontend and backend, you have to delete cache in browser to access the site again. 

.htaccess code is : 

Hello everyone,

I am trying to install Leaflet but it seems not to be working correctly. In Drupal, I am seeing it as a display type, so I can choose Leaflet, but the map is not being shown on the post. I have checked in the Status report section and noticed that it has problems detecting the version of Leaflet which I believe is the reason for the first problem. 

An AJAX error that results in a 403 message display, causes a user is unable to login. 
The user Masquerades itself as another role to check a feature. When checking it, gets the 403 message, the user try to navigate to another page and the site only shows the 403 page. The user tried to reconnect but was unable to even see the login page as only the 403 page was displayed.

I have been trying so long for this to work, before it always used to work with my old computer. With the new one I seem to be unable for it to work.

Composer works fine but Drush seems to not work, I am having an issue where:

We are in the process of migrating from Drupal 7 to Drupal 9. We have a main website and some subsites. We are doing a phased approach, migrating the main site first and then the subsites. We plan to have the  search functionality of the main website (to be in Drupal 9) still be able to index and provide search results from the content on the Drupal 7 sites.