Is there an expiation or repentance for breaking an agreement between brothers upon which “Al-Fatihah was recited,” and is this considered a cause of problems in life?

You are not required to offer expiation for retracting an agreement confirmed by the Fatihah. Any affliction that has befallen you may be due to other sins, for it has been said: "No affliction descends except due to a sin, and none is removed except through repentance." It was narrated from the Prophet, peace and blessings be upon him, who narrated from his Lord: "O My servants, these are but your deeds that I record for you, then I will recompense you for them. So, whoever finds good, let him praise Allah, and whoever finds other than that, let him blame no one but himself." When an affliction befalls a believer, he should blame himself and turn to Allah with repentance and seeking forgiveness. Among the punishments for sins is that they remove blessings and bring about misfortunes. Therefore, you must renew your general repentance, frequently engage in remembrance of Allah and seeking forgiveness, and be persistent in supplication.