に際して・にさいして - Grammar Discussion

Grammar Points
#1

on the occasion of
at the time of

Structure

  • Noun + に際し / に際して
  • Verb[る] + に際し / に際して
  • Noun + に際しての + Noun

硬

[Extremely formal・Used in instructions, warnings, speeches, etc.・に際しまして form is also used, mostly in speech]

[Similar to にあたって, に際して usually has to do with some sort of preparation for a future event]

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#2

For this example,

「ローレンス様とのご契約に際しまして、当方の契約書に誤りがございましたことをお詫び申し上げます。」

Why can の際に not be accepted in place of に際して? It seems to me like it should be acceptable or at least give orange text instead of completely wrong. If I’m missing nuance please tell me how that would change the meaning.