What is happening:
When I was doing the Rehearsal Roundup Quest, I could not do other quests that involve the same pony. For example, after giving the music sheet to Gypsum, I could not talk to Gypsum at all about the book delivery for A Book Too Far. Similarly I could not talk to Stoneshadow about The Letter of the Law vs The Spirit of the Law.
What should be happening:
I should still be able to talk to the ponies about other things, even if not about the Crystal Rehersal.
Steps to reproduce the issue:
Talk to Minister Spindle, accept The Letter of the Law vs The Spirit of the Law.
Accept the Rehearsal Roundup quest, give to Gypsum the music sheet.
Try to give Stoneshadow the music sheet, then try to talk to Stoneshadow.
Talk to Marshmellow Cream and accept A Book Too Far quest, try talking to Gypsum again.
LOE V0.1.8 on Macbook, Yosemite v10.10.5