While R21 and R22 are supported, they do not receive future updates beyond 4.4.0. R23, R24, and R25 users may technically use later Arnold versions (4.6.x), but 4.4.0 offers the best balance of speed and plugin compatibility with third-party tools like X-Particles and Insydium Bridge.
: It remains one of the few third-party plugins with full support for Team Render for both still frames and animations. Known Limitations in 4.4.0 Arnold Render 4.4.0 for Cinema 4D R21 R22 R23 R...
You are using an OSL shader written for a newer Arnold version. Fix: In R21 specifically, avoid OSL nodes labeled "Arnold 5/6." Stick to Standard Surface. Arnold Render 4