Looks to me like: Chestnut-flanked White-eye (Zosterops erythropleurus). I was at Plzen Zoo few months after your visit in 2016 and I saw both species. Celebes White-eye looks a little different. It was a very old individual with a damaged plumage.
Seems like you're right. I only saw this invidual and some oriental white-eyes. Figured this must have been the Sulawesi because it wasn't a Oriental - I wasn't aware they kept a 3rd species.
