The Rivash (siman 405), Rashba (Avodas HaKodesh 2:1), and Ritva (Eruvin 85b) maintain that mechitzos are essentially a matter of a d’Oraysa; therefore, even if the difficulty with the mechitzos would only be d’rabbanan, we are machmir. This is as opposed to eruvei chatzeiros, which is fundamentally a d’rabbanan in which case we are always meikil. In part 1 we demonstrated that (as long as the issue is d’rabbanan) most poskim maintain halachah k’divrei hameikil b’eruvin, even in issues regarding mechitzos.
The Maharsham (4:105) declares that even according to the Rivash, if the difficulty with the mechitzos is only d’rabbanan, we can be lenient, even if we cannot utilize halachah k’divrei hameikil b’eruvin, since it is a sfeika d’plugta bd’rabbanan (see also Maharsham, 8:58:5 and 9:18). Additionally, there are Rishonim and Achronim who maintain halachah k’divrei hameikil b’eruvin even regarding a matter of a d’Oraysa (Ravin, Hilchos Nida p. 138; Maharsham, 8:58:5, cites the Nachal Eshkol p. 169, and Eretz Tzvi, siman 86, cites Tosfas, Eruvin 44).