What are the times that should be followed for prayer in a European city: those mentioned in the mosque's daily schedule, or those found online, knowing that there is a difference of up to fifteen minutes?

Prayer times are determined by cosmic, Shar'i (Islamic legal) signs, and calendars that contradict these signs are not to be relied upon. If calendars differ and you are unable to determine the times yourself, then follow the calendar of the locality if its compilers are trustworthy Muslims. If such is not available, then rely on one of the calendars issued by trustworthy bodies, such as the Muslim World League or Al-Azhar. There is no harm in adopting what suits your study schedule, as the differences do not cause harm unless it becomes clear that the calendar contradicts the Shar'i signs.