In the NX Nastran Quick Reference Guide, Remark 1 for MATS1 states:
Thermo-elastic analysis with temperature-dependent material properties is available for isotropic linear and nonlinear elastic materials (TYPE = “NLELAST”) and anisotropic linear elastic materials. Four options for the constitutive relation exist. These options are listed in Table 15-3 along with the required bulk entries.
However, this remark does not apply to SOL 601. There are a special section of remarks for SOL 601 & 701. In this section, Remarks 5, 10 and 11 are applicable to your issue:
5. TID can reference a TABLEST entry for TYPE = “PLASTIC” but not for TYPE = “NLELAST”. 10. If the slope of the line joining the origin and second point in TABLES1 (for TYPE = “PLASTIC” and a given TID) is not equal to the value of E in MAT1 entry, the strain value at the second point will be adjusted accordingly. 11. If TYPE = “PLASTIC”, all tangent modulii ETi must satisfy the following: ◦HR = 1: ETi < E ETi can be negative when HR=1. Caution: Under certain modeling and loading conditions, a negative ETi may prevent a solution from converging and cause a failure. ◦HR = 2: 0.0001 * E < ETi < E ◦HR = 3: 0.0001 * E < ETi < E
You can specify temperature dependent plasticity for SOL 601. Your issue is probably related to the bounds on Et noted in Remark 11.