However, I cannot find anywhere else in my code where I use the word ´float´and this issue only occurred when I typed \usepackage{float}... I'll keep thinking about it...
you can follow the "hacking down" procedure from the minimal working example explanation (making a copy of your code and reduce it, so it can be posted here without unrelated content) and post the result here. I'm sure that when you post the code that prints 'float', we will find the cause.
Instead of using \begin{sidewaystable} I am now using \begin{landscape}. After all, this is a figure, not a table, so I guess latex went crazy not understanding why I was treating a figure like a table. When I do this, everything works nicely!