According to Microsoft OFX documentation, error 15500 means "Invalid Signon". Basically you are being connected to the OFX server, but your username/account number and or password/pin is incorrect.
Your OFX login information may not be the same as your bank's online banking signon information. Also, some banks require you to enable OFX Direct Connect before you can begin using it. Some larger banks require a fee to enable this feature - generally around $10.00 per month.
The first step to resolve this issue is to call your bank. Ask what you need to do to enable "direct connect for Quicken" (if you say Inzolo they likely won't know what you are talking about and say they don't support other 3rd party applications).
You can also try other applications that use OFX Direct Connect such as GnuCash (free) or a trial version of Quicken or Quickbooks to see if you are getting the same error. If you are getting the same error in Quicken for example, then you can confidently call your bank and tell them it is not working for you.