固陋頑迷な上司のせいで、新しい提案はすべて却下された。
Because of the stubborn, narrow-minded boss, all new proposals were rejected.
頑固 is the common word for stubbornness. 固陋頑迷 is a much stronger, literary yojijukugo implying not just stubbornness but also narrow-mindedness and poor judgment.
偏狭 means narrow-minded or intolerant, focusing on lack of breadth. 固陋頑迷 adds the element of obstinate refusal to change.
A yojijukugo combining 固陋 (narrow-minded, obstinate) and 頑迷 (stubborn, bigoted). The exact historical derivation is uncertain, but the compound intensifies the sense of stubborn narrow-mindedness.