That is strange, because like the error message says, if your php is set up with "open_basedir restriction in effect", it isn't allowed to create a temp file outside of specific directories. And if the configured upload_tmp_dir is outside of those directories, you'd get that error. But I think you would then also get the same error any time a file was uploaded, so I don't understand why it only happens in that one situation.
You might try temporarily disabling all plugins and see if that makes any difference.
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