Marc record import hangs when SearchEngine is set to ElasticSearch. To replicate: Make sure that ElasticSearch is running, and the SearchEngine syspref is set to ElasticSearch. Go to tools > stage marc records. Choose file, upload file, Click Stage Records for import. Click Import this batch into the catalog. The import hangs at 0%. I was able to import all records when SearchEngine was set to Zebra.
It turns out that I didn't have the 'elasticsearch' section set up in $KOHA_CONF... Command line tools warn about this; Staged Marc Management should too.
bug 19542 adds ES config to the about page, I think this serves as the warning error *** This bug has been marked as a duplicate of bug 19542 ***