Channel weighting of Mono channel in EBU R128 I am developing a loudness meter conforming to the EBU R128 recommendation. 
Currently, it works with stereo files but not with mono files.

In the ITU Recommendation, where the metering algorithms are defined, weightings for the individual audio channels are given (table 3, page 6). From that, I assumed that the weighting for a mono channel is 1.0 (0 dB).

As you can see in figure 1 on page 3, after applying k-filter, mean square and channel weighting to each individual channel, the resulting values of each individual channel are summed to one value. Logic dictates that when having two channels of the same audio signal, the value will be higher than when just having one.

As a consequence, a mono file will result in short-term loudness values, that are 3 LU lower in comparison to a stereo file with same content (on both L and R channels). This is clearly wrong, because established meters like "Klangfreund LUFS Meter" and "MeldaProductions MLoudness Analyzer" will output the same short-term loudness with both files.

How am I interpreting the algorithm definition wrong? How to deal with mono signals?

Side note: It seems that FFmpeg once had a very similar issue, but I could not find the solution to it.
Use a channel weight of 1.0 for your mono signal.

The weighting of 1.5 presented in ITU-R BS.1770 addresses the fact that a signal coming from behind is perceived louder by a human than if it is coming from the front (Reason: To better detect an enemy sneaking at you from behind). With a single mono signal this fact doesn't need to be addressed.

The 3 LU difference you mentioned also makes perfect sense from a physical point of view. One speaker in front of you compared to two (equal) speakers only provides half the energy and therefore the SPL will be lower by 3dB.

I'm honored that you use my LUFS Meter to compare to, but I'd recommend to also use the test files from ITU and EBU. The LUFS Meter manual contains the links to these test files.

Are you aware that the sources of the measurement algorithm of the LUFS Meter as well as some development notes are available? (Sadly I'm not allowed to post a third link... They are at docs/developmentNotes/)

Disclaimer: I'm the developer at Klangfreund.