I aways get a 504 error. According to a plugin support it is a database error.
I've deactivated many plugins in your backend to find a reason that is causing this issue.
And in the log i have found the error that breaks the import see: [login to view URL]
So this problem is related with your database.
In the google i found such tip for the issue: [login to view URL]
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Did you take a backup of the current database? That forum doesn't really offer much support than a theoratical approach. I am online. I look forward to hearing from you regards, Codinglogics