I see this error in the console of an application with many form fields, and I have no idea what it's about:
The specified value "NaN" is not a valid number. The value must match to the following regular expression: -?(\d+|\d+.\d+|.\d+)([eE][-+]?\d+)?
Aside from the type ("must match to" -> "must match"), the error could be more useful if it named that hard-to-read regular expression ("number" in this case), and, more importantly, if it said which field (or whatever it is Webix didn't like) had that value. For example:
The specified value "Nan" for custId should be a number