The first sentence is incorrect, but on account of de having no business being there whatsoever. Sembler is a verb that does not take a preposition before the following infinitive (it functions much like an English modal in that regard).

The second sentence is not incorrect, but strikes as a strongly elevated or even dated literary style.

It is common for most adverbs of scale or manner to be placed between the auxiliary and participle of a compound tense verb. I believe the same adverbs may occur between a modal verb and its auxiliary (whereas in English it depends on the verb, and different auxiliaries and semi-auxiliaries may allow different adverbs).

Certain adverbs (such as mieux, assez, beaucoup, trop, bien) as well as indefinite pronouns (such as tout and rien) commonly occur between a preposition and the verb, but personal pronouns and other adverbs (such as encore in your example) make for a markedly literary/poetic usage bordering on archaic.

Grevisse (Le Bon Usage 14th ed., §684 a) confusingly discusses this in the sections about pronoun placement rather than adverb placement (§§971-5) and I would have entirely missed without a cross-reference.

En effet :

“Tu me sembles de bien connaître …” sonne faux, tu devrais dire, “Tu me sembles bien connaître”.

“Je serais très heureux d’encore vous aider !” Sérieusement : c’est compréhensible, mais de cette manière, on pourrait penser que tu insistes sur le mot ‘encore’.

“Je serais très heureux de vous aider encore !” C’est mieux, ça fait plus poli, mais le mot “encore” insiste bien sur le fait que tu aides la personne une fois de plus…