NinjaTrader: “Provider already has a running connection”

The full wording is usually along the lines of: Provider ‘Rithmic for NinjaTrader Brokerage’ already has a running connection.
It has two quite different causes, and the fix depends entirely on which one you have.
Cause one: a duplicate saved connection
You have two connection definitions in NinjaTrader pointing at the same provider. This happens easily, usually after re-adding a connection to fix something else rather than editing the one already there.
Open the Control Center, go to Connections, then Configure. Look for two entries against the same provider and remove the one you are not using. If Configure is not in that menu at all, Multi-Provider mode is switched off, which is a separate problem with its own fix.
Cause two: you are opening a second Rithmic firm
If the connection you are trying to open belongs to a different prop firm from the one already connected, there is no duplicate to delete. You have hit the platform restriction.
NinjaTrader marks its Rithmic connections as one at a time. It does not matter that the two firms are separate companies with separate logins. The platform treats them as the same provider and refuses the second.
That restriction is on the NinjaTrader side, not Rithmic’s, which is why the same two accounts will happily run together on other platforms. What it takes to run them simultaneously is covered in running multiple Rithmic connections, and lifting it is what Unlimited Prop Firm Connections does.
Telling them apart
Ask one question: is the connection you are trying to open pointing at the same account as the one already running? If yes, it is a duplicate, delete it. If no, it is the platform limit and no amount of tidying the connection list will change it.
Affordable Indicators, Inc. is not affiliated with or endorsed by Rithmic, NinjaTrader, or any prop firm or broker named here; your firm’s and broker’s own terms govern how you may use your account, so check with them first.