Thank you for the post. Based on your description, it sounds like you are hitting the PHP memory limit. If this is TestRail Server, you can increase the PHP memory limit by editing your php.ini file. Instructions are located here:
You can also opt to set this to unlimited. To do this you would use the following: -1
If this is TestRail Cloud and you are still having this issue, we would recommend breaking up your report in to several smaller reports.