Now that's strange. It looks like a bug. I'll try to reproduce it later.
However, even if you didn't come across this error, you can not use synchronization. I mean, synchronization makes a snapshot of all volumes at a point of time to keep the data consistent. If you go the way of making several conversions volume per volume, there is no way to ensure this consistency.
I think you can try with one conversion as long as the number of volumes is no more than 9 (I see in a log they are 9) now that the ESX is working fine. If they are still too many, is it possible to exclude only 1 or 2 that don't need sync and which data can be copied later manually?