Elke Trautwein 

I suggest you try sending the SDLXLIFF files only if you have problems with the packages.  We can really only help if the translator provides more details, and also the error stack in full.

If possible you should try upgrading to make sure you are using the latest version and are compatible with your translators.  You could also consider working with a plugin such as the XLIFF Manager as this would allow you to avoid complications of differences between versions and still manage the updating of your projects quite easily.  This is a very good tool not only for working between Studio versions but also working with users of other CAT tools too.

As an agency it would make your workflow more robust, although you would lose some of the benefits of all working in SDLXLIFF files.  So based on what sort of feedback you get from your translators and what features you use in Studio, the XLIFF Manager could be a problem free solution to simply sending files and getting translated content back.