I am using the full Nittro build (nittro-full-2.0.64.zip downloaded from https://www.nittro.org/download) and I encountered this error too. I disabled everything that is not Nittro and still got it.
I experienced it exactly as described by @mabezdek. On click of the submit button, the span containing the error element gets added but is immediately removed somehow.
I found that if I enabled the live form validation using the data-validation-mode="live" attribute, everything behaves as expected. Only thing I noticed with the live validation is that if multiple inputs are required and empty, the error message appears only under the first one, but that might be a feature.