I am having two types of errors (the photo is attached) when I do lateral bending beyond 20 degrees and torsion beyond 55 degrees (1) too many iterations; and (2) too small steps . Do both errors affect the results of the calculations and analysis of reaction forces and joint angles? Are the results different if I used movements with angles less than the previously stated?
Yes, these errors indicate that the system could not find equilibrium for some of the timesteps within chosen tolerances. This typically happens when the system lacks stability or solver settings are not good enough to reach the solution within given limits. And the result of such time-step will be "the best guess" with some errors. These errors sometimes happen in the middle of the simulation and eventually "the best guess" leads to a good solution for the remaining timesteps.
You can try smaller angles and the default values are at 20 degrees, which should work (I presume you use SpineFixationWithFDK model). The torsion beyond 20-30 degrees is not a very realistic motion - these large RoM will be reached through rotating pelvis at the same time (try twisting your torso while sitting on a chair with fixed pelvis).
Please also check our wiki on FDK and revisit tutorials: