No, you are not permitted to contribute to a SEP IRA. With respect to a 403(b) you are considered self-employed and your retirement contributions are limited to $58,000 with respect to all of your self-employment income. You've already reached that limit with the contributions to the 403(b).
This is different than the case where the W-2 employer's plan is instead a 401(k). Contributions to a 401(k) in that case are not considered to be self-employed retirement contributions, so if this was instead a 401(k), you could make a SEP contribution even if the contributions to the 401(k) totaled $58,000.
It's not surprising that you have gotten different answers because it's easy to forget the special provision in the tax code (CFR 1.415(f)-1(f)(1)) that makes the 403(b) contributions be treated as contributions made by you as self-employed, Since both the contributions to the 403(b) and the SEP would both be treated as being made by you as self-employed, the combined amount is subject to a single $58,000 limit.
