erinja:To me, a kongreso is not a location, period..In PMEG it also says that you can't say 'renkonten'..

To me it depends on how strongly people are actually identifying an event with the place. This identification would be a lot stronger with a Congress than a renkonto with an individual. In Rotterdam in 2008, certainly by the end of the week anyone would have known what kongresen meant.

No less an Esperantist than Claude Piron would support this use. He uses kongresen in his article on the evolution of Esperanto. He writes: "A beginner in Esperanto will express the idea 'I'll go to the convention by train' saying Mi iros al la kongreso per trajno, when a more mature Esperantist will say Trajne mi alkongresos or Mi iros kongresen trajne."

erinja:To me, a kongreso is not a location, period.

I agree entirely. Clearly a congress is not a location, it is as you say an event that happens within a location. My point was just that kongreso is sufficiently evocative of the idea of a location to be able to function that way. We do this in English when we say something like "I'm going to congress", or "I'm going to school". Here neither "school" nor "congress" necessarily refer to specific schools or congresses, or even their respective locations; they just evoke the idea of those things as a "general place" that may be travelled toward, visited or attended.

Similarly, a bed is not a location, it's a physical object. Yet it has the idea of location when we say "I'm going to bed" or "mi iras liten", because "bed" in this case conveys the general notion of "place where sleeping takes place". I beleive the proliferation of kongresen and other similar words among experienced Esperantists and their obvious comprehensibility is enough to confirm that things can work this way too in Esperanto.

erinja:In PMEG it also says that you can't say 'renkonten'. Presumably this meeting is happening in a fixed place that the speaker or the listener is aware of, but it doesn't mean you can use the -n ending.

As I see it renkont' is an action and as such it would be meaningless to speak of it as a location with "renkonten". In any case, I wouldn't consider a meeting to be anywhere near as suggestive of location as a congress.

Miland:To me it depends on how strongly people are actually identifying an event with the place.

Exactly my point, and I think a good way of putting it. If people do make that identification then to all intents and purposes that is the word's meaning regardless of the official definition or anything we may want it to mean. Suffice to say I'm convinced kongreso can and often does function that way in the minds of a great many speakers. Certainly I've seen nothing to suggest otherwise.