A new FAQ was added to the Portal FAQs related to the submission system (and a news item referring to that). It can help to avoid some of the basic issues.
FAQ item with best practices for submitting proposal – https://ec.europa.eu/info/funding-tenders/opportunities/portal/screen/support/faq/17221
News item referring users to the above FAQ item – https://ec.europa.eu/info/funding-tenders/opportunities/portal/screen/support/news#17233
