Encoding::encode: source UTF-8 encoding declared, but ....

I use Plesk 12.0.18 on linux...

I have the following problem in the backup-manager with this message:

<object name="domain_name.de" type="hosting">
<message severity="warning" id="f55002eb-4df0-457b-895a-7d3c980a9f15" code="msgtext">
<description>Encoding::encode: source UTF-8 encoding declared, but invalid UTF-8 sequence occured. Falling back to ISO-8859-1</description>
</message>


then I found the following solution in the internet:

Make sure that there are no non ASCII...

Encoding::encode: source UTF-8 encoding declared, but ....