Poedit is giving an warning message:
9:48:02: ./campaignmonitor.php:1061: warning: Empty msgid. It is reserved by GNU gettext:
9:48:02: gettext("") returns the header entry with
9:48:02: meta information, not the empty string.
Line 1061:
$str = "<select name='$field_name' id='$field_name'><option value=''>" . __("", "gravityformscampaignmonitor") . "</option>";
Translating an empty string seems not very useful. Maybe you can fix this issue in the next release.