WordPress Migrator "Uncaught PharException: unable to open new phar" error
This message appears if your hosting account has reached its disk space limit while the plugin archived your website’s content for transfer or that the archive file has reached the maximum allowed file size set for your hosting.
Contact your hosting provider to increase the disk space and maximum file size limits for your account to resolve this problem.
If these limits cannot be increased then you should consider removing some of the content on your website so that when archived the content would not reach the set limitations for your hosting.
In some cases the error may appear if the permissions of the folders for your website are below 755. Make sure that the permissions for the folder your website is in and all subsequent folders are set to 755.