ESP test failure and calibration + profiling still working is quite common. One theory is that the test has flaws. I guess that there could be a time limit on the ESP like the normal i1 spectrometers should be factory calibrated in time. Price of a new ESP or a i1 returned to X-rite for calibration is roughly the same.
If the unit fails to calibrate and profile, one should make sure that it is not a glitch in the connection between system and printer, A restart of the printer did help when I had problems like that. Driver and software issues could give similar errors.
If it is a hardware issue after a repair (belt) and no problems with the ESP before, you could check the ribbon cable connection that had to be uncoupled for the belt repair, commands and measuring data must go through that connection. The other options are the sliding door of the ESP being stuck due to ink drying in at the friction points. At the inside of that lid there must be a white reference patch to calibrate the ESP on in my opinion, not been there though. It could have yellowed in time or got filthy. The lens of the spectrometer could be filthy.
I did see a message where someone wrote that the ESP test expects a glossy paper to calibrate on, should be a very quality controlled and plane paper in that case. Why the ESP test does not ask for paper during the test is beyond me then. It could be the flaw mentioned at the top. Common sense says that a calibration patch is used on the sliding lid that protects the spectrometer lens too.
Met vriendelijke groet, Ernsthttp://www.pigment-print.com/spectralplots/spectrumviz_1.htm
December 2012, 500+ inkjet media white spectral plots.