Emergency 5 was not designed for such a high number of emergency vehicles, objects and simultaneous missions. This leads to the graphics engine eventually breaking down and the ground disappearing. Depending on the PC, this occurs faster on some, takes longer on some, and on some the error never occurs. Depending on which graphics card and CPU you have. Unfortunately there is nothing we can really do about it.
As of 1.0.0, you can press F7, in many cases, to restore your ground. However, this is not a bug fix. It's just an attempt to restore the floor so you can keep playing and not have to quit.
Generally, however, the following tip helps with most:
Set population density to 35% or lower (Settings > Graphics > Advanced).
Graphics settings max. to "HIGH
Depth of field: OFF
If it still occurs, you can lower the settings even further and/or set the general graphics settings even lower. You need to find the settings that work for you.
Linzing is smaller, so the settings can be higher.
Brandstedt is medium-large, so the settings can be between Linzing and Lüdenscheid.
Lüdenscheid is very large, which means you have to reduce the settings here a lot.