diff --git a/apps/kolab_guam/src/rules/kolab_guam_rule_filter_groupware.erl b/apps/kolab_guam/src/rules/kolab_guam_rule_filter_groupware.erl
--- a/apps/kolab_guam/src/rules/kolab_guam_rule_filter_groupware.erl
+++ b/apps/kolab_guam/src/rules/kolab_guam_rule_filter_groupware.erl
@@ -45,6 +45,7 @@
     filter_folders(Buffer, State);
 apply_to_server_message(_ImapSession, Buffer, State) -> { Buffer, State }.
 
+imap_data(blacklist, { error, _Reason }, State) -> State;
 imap_data(blacklist, Response, State) ->
     %TODO: we don't need Foo/Bar if we already have Foo, so filter folders-of-groupwarefolders
     Blacklist = lists:foldl(fun({ _Folder, [ { _Property, null } ]}, Acc) -> Acc;