Recently upgraded my system from Win10 Pro ver 1809 to ver 1903 and Quicken would not accept downloaded transactions from my bank or investment accounts.
Did not get any error message from Quicken but all Quicken showed was that there were no transaction updates downloaded for banking or investment accounts but prices were updated for the investment accounts. Seeing as how I didn't do any downloads for two months, I knew that was wrong. I deleted a checking account and started fresh opening a new account. Bank showed there were 807 transactions, Quicken said the same but only the first transaction was installed. I checked the downloaded file using Notepad and there were hundreds of entries...
Researching the problem found that the culprit was most likely a Folder Protection problem and solutions ranged from reinstalling Quicken to moving it to a different folder to you name it.
The Fix: Decided to download and reinstall the last Quicken update patch and that cured the problem.