What I believe that people who have this problem are experiencing is that they are trying to play a 6.0 alpha save with the 7.0 game. They don't appear to be compatible.
The simple solutions are to download the clean saves from page 152 (posted 2/2/2020) or sign up for the Celaviegroup Patreon for some direct help from the Dev.
In the file main.rpy, line 7532, the developer checks if the variable eventhack6_1 equals 1.
If it does the game should show you a black screen with big white letters letting you know your savegame is from an illegally downloaded version of the game and then quit.
if eventhack6_1 == 1:
show black
show text "{size=100} You have played an illegal alpha version of Room for Rent. In order to continue you must contact CelaVieGroup {/size}" with dissolve
$ rfr_pause(1, hard = True)
$ renpy.quit()
Problem is, at least in my case, that it doesn't show the black screen, so I'm unable to see the warning message and the game just quits.
If you change the code from "show black" to "scene black" then it works as the developer intended.
A bug?
But well, if you have gotten there you might as well force the variable to zero before the check and continue playing with your old savegame...