Parse error, but still yields a correct AST about langium

[spoenemann]

The error is likely due to the way how you defined EQUALS (with positive lookahead). Changing to a simple '=' keyword fixes it:

NamedArgument:
    name=NAME '=' value=Value;

Value returns string:
    STRING | NUMBER | NAME | SELECTOR | BOOL_TRUE | BOOL_FALSE;

Here I also extracted the value to a string typed rule.

[msujew]

Note that there are actually two red squiggles (right after another):

The first one indicates that the = sign couldn't be lexed (due to the issue outlined above by @spoenemann), while the other indicates that EQUALS had to be skipped in order to parse the input.

[rolandzwaga]

Argh, thank you gentlemen, the problem was indeed the defintion of my terminal. The grammar I posted was a simplified version of my real grammar. I now see that I ordered some terminals wrong there. In an effort to try and fix the problem I was trying out all sorts of things and had added the look-aheads.
Not realising this actually just made matters worse...

Thank you for your time!