The vanishing error is something we deployed - clicking once on an error will hide the whole debug window. It is quite useful as we are developing epesi, but I can see it doesn't really make it easy to copy the error message. The easiest to do right now is to screenshot the error. It's possible to select the text without it vanishing, just not very straightforward.
I think I'll change the way errors behave, hopefully pushing it to the next release.
Now, as for the error itself - not long ago we've been going through some pretty heavy drag'n'drop testing in calendar and we didn't find any issue with any type of the event.
If you happen to come across this error again, we'd be grateful for more details as to what exactly you did and what was the error message.
Right now there's nothing we can do really, I'm afraid.
Kind regards,
Arek