It is often said that Porta clausa est can have two readings depending on the categorial nature of the participle: verbal (cf. clauditur/clausa est) or adjectival (cf. clausa est/clausa fuit), which in turn corresponds to a semantic difference between process and result/state, respectively. For example, in Spanish this distinction is also made clear by using different auxiliaries, ser vs. estar: cf. Sp. La puerta fue cerrada (sc. by someone) /La puerta estuvo cerrada (*/??by someone). A typical (although recently argued to be a bit problematic) test to distinguish verbal passives from adjectival ones is that the former can be used with an agentive by-phrase, while the latter can’t. As far as I know, this test has also been assumed to hold for Latin: cf. verbal passive (porta {clauditur/clausa est} a Gaio) with adjectival passive (cf. porta clausa {est/fuit} */??a Gaio).
Given the previous (typical?) approach, I think that the learner of Latin cannot often know if there is still a third reading available for porta clausa est or porta fracta est, the one corresponding to the so-called “anticausative” variant, e.g., 'The door broke' (e.g., cf. the so-called "causative alternation" in English: {John/The strong winds} broke the door (causative variant) vs. The door broke (anticausative variant)).
If so, porta fracta est can be said to have the following three (rather than only the typical two) readings:
1: verbal passive: the door was broken/has been broken (by someone).
2: adjectival passive: the door is broken (*by someone; *on this adjectival reading!).
3: anticausative: the door broke (*by someone). (NB: the use of * means ungrammatical).
Consider the following example, which can be taken as (good? What do you think?) evidence of the anticausative reading (which is, as noted above, different from the processive verbal passive reading (to be broken by someone) and from the resultative/stative adjectival one (to be broken *by someone)).
Postquam omnis res mea Ianum ad medium fracta est, aliena negotia curo, excussus propriis. (Hor. Sat. II 3.18-20)
‘After all my business collapsed at the central arcade of Janus…’
Now here is my question: Which is the nature of the participle in the anticausative reading of fracta est? Cf. the verbal-adjectival distinction drawn above. I would say that in this case the participle has a verbal nature (like in the first verbal passive reading above) but perhaps I’m wrong. More examples of (clearer) anticausative readings of periphrastic forms would also be greatly appreciated.
NB: here I’m mainly interested in the ambiguity of fracta/clausa est rather than in the one also involved in non-periphrastic forms like frangitur/clauditur, which can also have different interpretations. When dealing with non-periphrastic forms, further distinctions become relevant (e.g., cf. frangitur/se frangit, the pronominal form becoming even much more frequent in Late Latin).
What follows contains a relevant modification I should make thanks to the very useful feedback I’ve received from the commentators. Interestingly, I've been advised by the majority of them on the convenience of distinguishing what one could call the "uniformity" of the structure (e.g., the one corresponding to Porta clausa est) from its possible associated meanings (and "nuances", in Joonas's terms). Here is my personal view, which, against perhaps the opinion of some commentators, does crucially separate the three basic "meanings" (into types) from "nuances" (into subtypes). I consider that fuzziness is not involved at the type level (perhaps things are different at the subtype level...). For the sake of exposition, I also consider that it can be useful to use typical semantic predicates like CAUSE, BECOME, and STATE (which have been used in the specialized literature on lexical decomposition of verbs; e.g., Brutus killed Caesar can be decomposed into 'Brutus caused Caesar to become dead'). Next Porta clausa est can be analyzed into the following types and subtypes (NB: of course, some commentators and I disagree on the more convenient "labels" and on the most appropriate "taxonomy" but after all what is really important/relevant is what is behind them).
Verbal passive: 'the door was caused to become closed' (by someone)
Anticausative/"middle": 'the door became closed'.
2.1. ‘the door became closed because there has been another previous/implicit event that caused the door to become closed’
2.2. ‘the door became closed by itself'.
Adjectival passive: ‘the door is closed’.
3.1. Resultative adjectival passive: 'the door is closed as a result of a previous event of closing'.
3.1.1 ‘the door is closed because there has been a previous event of causing the door to become closed’)
3.1.2. ‘the door is closed because there has been a previous event of just becoming closed’, i.e., with no agent involved.).
3.2. Stative adjectival passive: 'the door is closed (with no previous event of closing involved)'.
Notice that in this classification it is important to distinguish the three types from further subtypes/nuances. My impression is that the three basic types are clear, whereas some subtypes are perhaps not necessary. Some further observations/remarks/caveats/... could also be made but here I will limit myself to commenting a distinction that can be drawn within the domain of adjectival structures: cf. the resultative and the stative readings. For example, in Porta clausa est the resultative one is the most prominent one. However, the stative one is not excluded (e.g., imagine that the door was built closed). As for the adjectival reading of porta fracta est , in this case, due to our encyclopedic knowledge, the resultative one seems to be the only possible one.