Font conflict causes misbehaviour in Mathmode
The Texcode: [ \textrm{CIR} [ t ] = (g_{\textrm{loc}} * s) [ t ] ] will be displayed as:
I opened a question at tex.stackexchange https://tex.stackexchange.com/questions/696023/is-not-rendered-at-all-and-neq-as
they answered: The problem is that you are loading classicthesis with eulermath and then also loading lmodern after you loaded classicthesis. Now, removing lmodern will "change the font", but you are already specifying palatino and eulermath when loading classicthesis, so you've placed yourself in a position where there will be font conflicts no matter what. – Willie Wong
Willie Wong is absolutely right. classicthesis is not very flexible, if you want to change basic things like the font. Maybe it would be better to not use the package, but only some of the code (or implement the wanted layout in a different way, e.g. without using not recommended combinations of packages and classes)