I guess this is related to earlier trouble with series ordering. No idea if fixes to other ordering problem fix also this so I will enter new ticket for this. Changing order numbering of parts returns ISE on some cases. For example https://beta.musicbrainz.org/series/1e174a5e-2955-4013-b8c3-1641f34f26cb try changing 1 to 10 and 10 to 1. No edits are entered and you see ISE.