Hi Arne, there have been a lot of processing issues over the past several months. The system is being rebuilt to accommodate the increased volume of data, but in the meantime there have been fairly consistent processing backlogs. Glancing at some stations in Ontario it looks like most are about 4 weeks behind at the moment.
That said, you shouldn’t be continuously seeing that error and should be able to upload your data. I just checked now and am able to access the upload page and didn’t have any issue uploading data from about 8 stations last week and notice my colleague here just uploaded data from 3 stations this morning. Manual uploads are currently only being accepted from 8 am to 8 pm Eastern time (noon to midnight UTC I believe) but there should be a notification of that on the upload page during that time, not an error message.