failed (UCS application failed; unknown cause.)

THE_BLUE Just because you can install it on the passive one without issues does not mean that will be the case for the active unit. I would attempt to get them to the same space amount first and after that I would ensure they both have the same amount of available code versions uploaded to them and installed in partitions. What I have seen in the past is that one unit would have images ready to install where the other would not which would cause a space issue. I have also seen each device have more paritions with than the other with code versions installed as well. Make sure they are exactly like for like and attempt the upgrade once more. Other option here would be to open an F5 TAC and see what they say.