I have 2 Macs in my office running Sierra 10.12.6 and Autocad 2015.  Recently I've set up a 3rd drafting station w/ new iMac thats running High Sierra 10.13.1.  Yesterday I downloaded Autocad LT 2017 on a 30 day trail basis.  issues I'm having are;

1] if I can open an existing file and modify it I can't save back and overwrite previous file in directory - getting an error message stating "One or more objects in this drawing cannot be saved to the specified format.  The operation was not completed and no file was created".

2] if I open and modify drawing and save it somewhere else on the file manager I can only open it with Autocad LT even though I "saved as" earlier autocad/autocad lt version

is this a compatibility issue between Autocad and Autocad LT ?  

An issue between Sierra and High Sierra ?

or something else ?  

any response to help will be appreciated - thanks