Import Contribution - last step no longer goes to 'Summary / Error' report
How large is your import? It seems to happen on larger imports and this has occurred on and off in Civi for some time in my experience. Something is causing it to hang. 

You can still get to the summary page by replacing Preview with Summary in the URL. That will show you any records that could not be imported, etc. You can also look at MySQL and check that the import is done if the temporary table (civicrm_import_job...) is no longer there. If the table is still there, search status = NEW to find records not imported. 

Yes, this is a workaround but it will get you on your way for the time-being.