Thanks for picking up this one.
It has been reported here before, by both of us. But I couldn't track down the earlier references that clearly also passed Chris by.
Fortunately, it isn't quite so persistent here. And I have never found any obvious pattern.
I do not, though, see it only with GMail. Indeed, I cannot recall seeing it there. For me it is with a different IMAP service.
I have just found the conversation where we discussed this before:
X1 Discussion • View topic - Gmail data source error
You even provided a grab of the error:

Note that Rhonda also said:
Several X1 folks volunteered to look into it, back in September. But, as I said earlier in this discussion, I have yet to see a meaningful explanation.
The last contribution, at the end of November, was when Jogy suggested that the new beta version would squelch the bug. Nope.