Why I see no safety distance calculated after making a measurement?

You need to take at least 10 measurements to calculate safety distance. See User's Manual Safety Calculations section.

Why there is sometimes +3 standard deviation text underline and sometimes maximum time text is underline?

Underline identify current value used in safety distance calculation depending what value is larger. See User's Manual Safety Calculations section.

Sometimes the software seems to not respond to the click on a Start button.

When the device is in armed mode, you cannot send another command and thus the start button and release setup is disabled until device finish measurement and measured value is received.