personal reasons; personal circumstances
A formal set phrase used especially in resignation letters, official statements, and polite explanations for leaving a job or position. It avoids giving specific details.
I will be resigning for personal reasons.
He was absent from the meeting due to personal circumstances.
A more general and less formal way to say 'personal reasons'. 一身上の都合 is a fixed, formal phrase used in official contexts.
A set phrase composed of 一身上 (personal, individual) + の + 都合 (circumstances, convenience). The exact historical origin is uncertain, but it has long been used in formal Japanese as a polite, non-specific reason for leaving a position.