The U.S. State Department released the 2015 Compliance Report. I don't think anyone was counting on the State Department to release more information about the substance of its claim of Russia's non-compliance with the INF treaty. The State fully met everyone's expectations - there are nothing specific there.
Having said that, a simple track change comparison with the 2014 Compliance Report shows some interesting changes. First of all, the report adds two more articles of the treaty to its "compliance analysis":
Paragraph 7 of Article VII provides that if a launcher has been tested for launching a GLCM, all launchers of that type shall be considered to be launchers of that type of GLCM.
Paragraph 8 of Article VII of the INF Treaty provides that if a launcher has contained or launched a particular type of GLCM, all launchers of that type shall be considered to be launchers of that type of GLCM.
This may not be very significant, but it does seem to support my guess that the violation is a technicality that involved a test from a "wrong" kind of a launcher:
the violation is largely a technicality having to do with the fact that the INF Treaty requires that any missile that is not a GLCM covered by the treaty should be test-launched from a "fixed land-based launcher which is used solely for test purposes and which is distinguishable from GLCM launchers." Strictly speaking, a test of a SLCM (with a range of more than 500 km) from a road-mobile launcher would mean that this SLCM would qualify as a GLCM and therefore will be a treaty violation.
The difference between a "fixed land-based" and "mobile" launcher is rather thin - for example, as far as I can tell, a cruise missile can be tested from some kind of a truck, but if that truck has its wheels taken off it could conceivably classified as a "fixed" launcher. But if the wheels are there, it would become mobile (even if it doesn't move much).
So, I think the evidence that we have still suggests that it was a sea-launched cruise missile that was tested from a truck that happened to have wheels. It is a violation of the letter of the treaty of course, but not a very dramatic one.
Politically, of course, it is a rather big deal, especially if it turns out that RS-26 is in fact an intermediate-range missile. Unlike the cruise missile, this one would be a clear violation of the spirit, but not the letter of the INF Treaty, making it is difficult to bring a formal complaint. But to leave these two developments without a response is not really an option for the United States. so it went after the truck.