What is the correct solution to the problem of the 'Isha prayer time being delayed until 0:15 at night in summer in Europe, which has led to a division among Muslims between those who combine Maghrib and 'Isha with an early combination and those who pray 'Isha an hour and a half after Maghrib?

Both parties are mistaken. You must pray each prayer at its appointed time, no matter how late the sunset or how short the time between Isha and Fajr. It is not permissible to combine two prayers for this reason, nor to estimate the time of Isha based on personal ijtihad. It is obligatory for one who is ignorant of the ruling on an issue to ask the people of knowledge, for Allah the Almighty says: "So ask the people of the message if you do not know." Indeed, knowledge precedes speech and action.