there was something in the mzML that was not acceptable by Xcmsonline. I reexported as mzXML and it seems to have worked (at least it got past the File Check phase).
When I try to run them in XCMSOnline, I get "ERROR, WRONG_POLARITY"
An incorrect polarity parameter was selected in the parameter settings for the datafiles being run. Please check the polarity of the uploaded files and rerun with the correct polarity settings.
My XCMS parameters are set to positive using HPLC/Orbitrap.