Hi,
To be perfectly clear, the Ultimaker 2+ and every other Ultimaker printer is designed to be used with 2.85 mm filament. Not being able to print 1.75 mm filament is not an error it's a design choice.
A printer designed for 1.75 mm filament will never be able to print 2.85mm filament, it simply wont fit. The same is true for a 2.85 mm printer like the Ultimaker, but for other reasons. Please use the correct diameter filament for your printer.
Because the Ultimaker is open source people with enough know-how will be able to adapt it for use with 1.75 mm filament. However, this will require some serious modification to the printer. The kind of modifications @SandervG is talking about
Best regards,
Johan