Dear Emanuele,
Were you ever able to solve this problem? Which version of AeroDyn v15 are you using? Does upgrading to the latest version of FAST v8 help?
This error means that the blade-element/momentum theory (BEMT) solution algorithm of AeroDyn v15 cannot find a valid solution. The error has been discussed several times on this forum. Please use “Search…” in the upper-right corner to see how other users have solved similar problems in the past.
From the following forum topic, we are aware of numerical problem in the BEMT convergence algorithm of AeroDyn v15 when Vx gets very close to zero, but your Vx is quite a bit lower than zero: AeroDyn v15 convergence issues - #7 by Jason.Jonkman.
A negative Vx is a bit odd. Does the structural solution look satisfactory up until the point of failure, or is there perhaps a numerical instability whereby the structural motions become very large?
Best regards,