Sometimes you might find a validation error without any specific information. as in:
WARN 11:00:56,968 The attachment 'MMMMMMMMM.zip' does not exist at '/tmp/com.awnaba.projectconfigurator1493567880435/data/attachments/XXXXXXXXXXX'. It will not be imported. WARN 11:00:56,968 The attachment 'screenshot-1.png' does not exist at '/tmp/com.awnaba.projectconfigurator1493567880435/data/attachments/YYYYYYYYYYYY'. It will not be imported. WARN 11:00:56,968 The attachment 'screenshot-1.png' does not exist at '/tmp/com.awnaba.projectconfigurator1493567880435/data/attachments/ZZZZZZZZZZZZZ'. It will not be imported. INFO 11:00:56,969 Project Import: Validation errors were found. The import cannot continue.
You also examine the JIRA log file, but that does not throw any additional light on the issue. In our experience, this problem is usually caused by a value for a text field that exceeds the limit for maximum text field size at the destination. See point 3 in the Matching properties of both instances section of Atlassian's Preparing for migrating projects page.