I use photo mechanic a good amount. In the last week or so LR CC has started locking I import files. Almost seems like the sidecar .XMP (or something weird) is causing LRCC to lock up hard on Windows 8.1 (when data is edited, not left as default when imported) when I try to do an import. I thought clearing the Cache worked at one point in LRCC, but not this latest time.
I just imported a folder of 2000 photos that I shot last night and did not edit any ITPC data in PM, just imported. No issues. The folders I have had issues with were cropped and captioned in PM and contained less than 20 files.
LRCC version is 2015.1 with Camera Raw 9.1