لَمْ يَكُنْ مِنْ نِزَاعٍ فِي خِلَافَتِهِ، بَلْ عَنْ خَطَإٍ فِي الِاجْتِهَادِ
It was not a quarrel about his caliphate, but an error in ijtihād.
The particle bal rejects one explanation and introduces a different one. The first explanation would locate the conflict in opposition to the legitimacy of the caliphate; the second locates it in a disputed practical judgment.
The phrases “from a dispute” and “through an error” signal different causal descriptions. The contrast is interpretive, not a claim that the two expressions are synonymous.