Not "since we have no merits," but "where we have no merits" (in this one)

 "Placare domine, quaesumus, humilitatis nostrae precibus et hostis et, ubi nulla suppetunt suffragia meritorum, tuae nobis indulgentiae succurre praesidiis" (Corpus orationum no. 4246).

 "Placare domine, quaesumus, nostrae humilitatis precibus et hostiis et, ubi nulla suppetunt suffragia meritorum, tuae nobis indulgentiae succurre praesidiis" (Missale Romanum).

"Be pleased, O Lord, with our humble prayers and offerings, and, since we have no merits to plead our cause, come, we pray, to our rescue with the protection of your mercy" (Roman missal, 2010 translation).

     Super oblata, Second Sunday of Advent, Missale Romanum.  Present at various points in the 8th-century sacramentaries.

     Clearly, that "since," which stood out to me at Mass this morning, was too good to be true.  For I see no indication that ubi functions in that way in even the medieval Latin covered by the Dictionary of mediaeval Latin from British sources, let alone the classical.  That phrase should run not "since no votes or backings of merits—i.e. no judgments of merit—suffice," but rather "where no votes or backings of merits—i.e. no judgments as to merit—suffice."