We are migrating from a Zimbra 7.1.3 server to a zimbra 8.8.12 and we are using the Zextras Suite

The export on the source server goes well, but when trying to import the backup on the destination server with hte Zextras Backup zimletwe get the following error message:

Client: Administration,
,JavaScript Exception: Error processing SOAP response: Server returned an error. at Function.q.processResponse ( Zimlets-nodev_all.js.zgz.js:31744 ) at q.callback ( Zimlets-nodev_all.js.zgz.js:31724 ) at AjxCallback.run ( zimbraAdmin/:790 ) at ZmCsfeCommand._runCallback ( Zimbra_all.js.zgz:261 ) at AjxCallback.run ( zimbraAdmin/:790 ) at Function.AjxRpcRequest.__handleResponse ( Ajax_all.js.zgz:814 ) at XMLHttpRequest.__httpReq.onreadystatechange ( Ajax_all.js.zgz:789 )

Any help would be greatly appreciated.

Or any idea on how to best migrate 7.1.3 - 8.8.12?