Yes, I did figure it out. In addition to the Solo 401k contributions, I had also made contributions to a SEP IRA. The SEP IRA contributions and Employer Solo 401k contributions count towards the same tax-deductible retirement account contribution limits. I had maxed out my SEP contributions (based on my business income) and therefore the 401k employer contributions put me over the limit. Turbotax correctly will not let you double count.
