February 15, 1650
Nuremberg, Bavaria
Catholic Congress of Franconia

The Franconian Diet with approval from the Archbishops of Bamberg and Wurzburg:

Article 1: During an interval of ten years, reckoning from this day, no man will admitted into a monastery who has not reached his sixtieth year.

Article 2: All priests and curates not belonging to a religious house or chapter are to marry without delay.

Article 3: Any man is allowed to marry two wives; but husbands are advised, and they will often be reminded of it from the pulpit, that if the fate of two persons is entrusted to them, they must in turn behave discreetly and prudently, provide for them sufficiently in the first instance, and then take measures to prevent hatred springing up between them.

Here's a subreddit discussing this:
https://www.reddit.com/r/AskHistorians/comments/5gzgx8/on_february_14_1650_the_parliament_at_nürnberg/

In short, the Wikipedia page on polygyny has 6 refs for this, one is a 1790 journal with the text (saying 10 wives), 3 are probably references to the 1790 text (one is a general genealogy handbook, one seems to be a 1859 Protestant tract, probably anti-Catholic, and the third is a biography of Philip of Hesse). The remaining two are scholarly references. One says this body didn't meet during this time and there is no record in proper archives, and the other says the 1790 journal is a popular journal, not state archives, and this is a literary fantasy.

Even IF it were true, it would be a political act, not a Church act - the referenced archbishops were prince-bishops. The prince-bishop of Bamberg at that time spent 10 years as bishop-elect without being consecrated bishop. The prince-bishopric of Wurzburg in 1650 was held by the archbishop-elector of Maintz.

Now Philip of Hesse did live in bigamy, with the approval of some Lutheran theologians. (There is dispute whether Luther approved it.)

Early Lutheran theologians were against divorce, but apparently thought that the Bible did not explicitly forbid polygamy.

Luther said that monogamy is the standard under normal circuмstances, but extreme exceptions could exist. I can't find a reference, but I believe Luther gave as an example, a wife with leprosy. He apparently felt, among evil things, bigamy was better than divorce or adultery. He also said the state could not legalise polygamy in general.

I confess that I cannot forbid a person to marry several wives, for it does not contradict the Scripture. If a man wishes to marry more than one wife he should be asked whether he is satisfied in his conscience that he may do so in accordance with the word of God. In such a case the civil authority has nothing to do in the matter.

Philip Melanchthon said that Henry VIII had no need to risk schism to divorce his first wife, but could have married another.

Philip of Hesse married Christine of Saxony in 1523. He was 19 and she was a few days short of 18 at the time. Philip committed adultery within a "few weeks". (That sounds to me like he was unfit for marriage.)

After years of Philip living "constantly in a state of adultery and fornication" (per Luther), and with some degree of approval from Lutheran theologians and from his wife, he took a second "wife" in 1540. It is said that Philip found Christine ugly, but he had 7 children with her by 1540 and would have 3 more before she died in 1549. He had 9 children with his second "wife", 6 while Christine was alive.

Luther allegedly gave his advice in the confessional and did not formally acknowledge he had approved the 1540 marriage.