Deafult render plugin malfunction

hello dear friends,
im using an evaluation copy of rhiboceros 5 [64bit], once in a few days [occasionally] when i use the software there is an error message on the upper-left side of the screen that says : “Unable to load default render plug-in”.
while this massage appears all my toolbars disappears and after i try to open them, through “toolbar layout” at the “tools” tab it shows me and error message that “the rhino toolbars and menus plugin can not be loaded, but it requires to run the toolbar command”.

its the second time it occurs to me, the last time i just reinstalled the software.
anybody got a clue why it happens ? how can i solve this issue ?

i’ve added 2 screenshots to show the issue properly.