How to fix
Are you using Solar Balancing?
Try adding in more power from the grid. If your charger is constantly starting and stopping because there is not enough solar power, it may cause your car to end the transaction.
Do you have a 1-phase building and will you upgrade to 3-phase?
The charger may be wired in a special way (3 phases from 1) to prepare it for the coming upgrade, but not every car can handle that. Contact your installer and ask them if they wired the charger with 3 phases or just 1.
Do you have a (Home) Energy Management System?
If other systems are controlling the charger (or your car), this may interfere with Volt Time's load/solar management tools. Try just charging in Boost mode )without Peak Shaving or Solar Balancing).