Then it is a bug in your router which destroys the valid syntax of this line (mandated by the FTP specifications) and may cause FTP clients to complain and error out. Look for a firmware update, the problem may be known already.The double bracket shows only in ftptest.net, but not in the logs
No, that's not normal (and not desired). The router does port remapping. In the posted logs, the server tells it will listen for the data connection on port 5010. However, the router changes it into port 61781 for the outside world (the FTP tester in this case). This is bad behavior, as now the router must handle ALL conversions from one number to another. This will break sooner or later.Also note that the last two numbers differ in both cases, for some reason...
You said you added port mapping? Check the settings you made: You have to select the same internal ports as the external ones.