Errors occur when executing geoprocessing functions on UT data.The error message
Error: 00029: Cannot open <value>.
There is an technical article from Esri, which describes causes and a solution.
In case of UT it is caused by the fact, that the UT object class extension is only registered for 32 bit and thus not available for 64bit-geoprocessing in background.
Disable 64-bit background geoprocessing as described in the article above.