How come the decoder gives a question mark?
The problem I wrote about in the comments under the question regarding the waveform signal integrity has to do with the SCL waveform shown as below:



To be able to ensure that you have reliable communications with the target device it will be necessary to make sure that your SCL driver is able to actually pull the SCL signal right down to GND level and not have it nearly a volt above the GND level. The slave device will not work correctly if the clock signal only pulls down to 1V.

The depicted SCL waveform could be caused by a scope probe that has not been properly frequency compensated. 

It is also possible that you probed the signal without the GND lead of the scope connected near the signal source.

It is also possible that you have a truly bad scope probe or one with bad internal contacts in the connectors. 

Since the waveform you show for the SDA looks so clean you may want to capture the waveforms with the probes swapped to see if the SCL displays properly with the other scope probe.

If the SCL waveform shown on the scope shot is truly an accurate depiction of the waveform on the board then some work needs to be done to correct the problem.