I'm a feminist but... is a silencing tactic used by someone who is or claims to be a feminist or ally, but tries to silence a discussion by claiming that it is taking feminism too far or too seriously. It's often followed by these other silencing arguments:
It often has overtones of concern trolling.
There's a variant of this tactic in which the person using it implies or threatens in some way to abandon their feminism over a particular point: "I think of myself as a feminist but..." "I used to consider myself a feminist but..."
This doesn't imply that feminists don't or shouldn't genuinely disagree about feminist interpretations of events, the problem is with treating every disagreement or challenge as an enormous problem with either your own feminism or feminism in general and using threats to abandon your ally status entirely in order to silence others, particularly if they find a given event or community more problematic than you do.