I thought I might add the relevant part of §104 in Lambert's Phänomenologie (his emphasis):

“Das Wahre in den Begriffen kömmt darauf an, daß sie möglich seyen, und ein für sich gedenkbares Ganzes vorstellen. Dieses macht sie richtig und vollständig. Die Merkmale, die man zusammennimmt, müssen sich können zusammennehmen lassen, und es muß keines wegbleiben, das zu den übrigen mit gehört, damit keine Lücke in dem Begriff bleibe. Was hiebey übersehen oder aus der Acht gelassen wird, das macht den Begriff nur scheinbar, oder dem Schein nach richtig, und es ist ein bloßer Schein, so oft in dem Begriffe noch entweder unbemerkte Widersprüche oder unbemerkte Lücken sind, oder gar beydes vorkömmt.”

My translation:

“The truth in the concepts depends on their possibility and that they represent a conceivable whole. This makes them correct and complete. The criteria that are combined must admit combination and none must be left out which belongs to the others, so that there be no gap remaining in the concept. What is thereby overlooked or goes unnoticed makes the concept only apparent, or correct by appearance, and it is a mere appearance as long as there are unperceived contradictions or unperceived gaps in the concept, or even both.”

In the broader context of Lambert’s phenomenology ‘apparent(ly)’ and ‘appearance’ are suitable translations of ‘scheinbar’ and ‘Schein’, though in the context of the issue at hand and with a perspective at the Vienna Circle ‘pseudo’ would be more appropriate. One may read the comma after ‘scheinbar’ in a way that distributes ‘richtig’ to ‘scheinbar’, too. Then Lambert would be talking not about apparent concepts (“scheinbare Begriffe”, i. e. pseudoconcepts) but about apparently correct concepts (“scheinbar richtige Begriffe”, i. e. not necessarily pseudoconcepts). However, I see no strong reasons why the first reading should be less plausible than the second. Therefore part of the quoted section can be seen as a first characterization of pseudoconcepts. To that purpose Lambert invokes the notions of possibility, representation, (defining) criteria, their combinability among others. The latter two notions fit very well with some approaches by the Vienna Circle: The defining criteria of a concept can be viewed as a prerequisite of the reduction of that concept, foreshadowing the reductionism of the empiricists (Lambert’s own empirico-reductionist inclinations are evidenced in his Semiotik, §136); the combinability of criteria may be reframed as syntactical well-formedness.

An interesting question, Moritz, since pseudoconcepts probably form a very large subset of 'concepts' in several branches of science and philosophy. Leibniz would obviously had something to say about this had he thought it polite and constructive to discuss it. Thinking about his main texts I suspect he preferred to point out error or contradiction rather than insinuate that others had not even formed their concepts properly. His style was more 'esteemed sir, many thanks for your interest, but you are wrong.' Leibniz wrote in several languages and not in English as far as I know. So I would have trouble knowing what to trawl for in French, Latin or German.

He certainly points out where he thinks this sort of error occurs but my suspicion is that he does not express it quite in these terms. 

?Shopenhauer.

Dear Moritz

I think it may be interesting the case of philosopher Carlos Vaz Ferreira of Uruguay, who in the very early XXth century raised the issue of pseudo-problems, which he called "paralogisms of false opposition".

The case can be interesting not so much time priority, and its geographical marginality. Being a fairly away from analytic philosophy and general philosophy German or English author, it has been rediscovered in recent years as if it were an original history of the philosophy of language.

You can find many of his works in the Internet , and critical studies in Spain and Uruguay, mostly.

From Vaz Ferreira, you can see, for example: "Lógica Viva" ("Logic alive") for a general treatment with many examples, and "Los problemas de la libertad y los del determinismo" ("The problems of freedom and determinism") in which he applies his method of analysis to an important philosophical problem). In Spain, you can look for Luis Vega Reñon, Manuel Atienza; in Uruguay see works of José Seoane, Miguel Andreoli; en Costa Rica: Enrique Pedro Haba, etc.

See Alfred Sidgwick, "Propositions with a View to Proof", Mind, Vol. 8, No. 29 (Jan., 1883), pp. 22-47.

On p. 24 Sidgwick identifies three kinds of propositions that he regards as "unreal propositions" or "pseudo-propositions", viz. tautologies, contradictions, and those in which "any term used ... fails to convey intelligible meaning."