According to this, Norway has a "joint collecting society in Norway for musicians, performing artists and phonogram producers", Gramo, that performing artists must pay a levy to Gramo when publicly performing works that are in the public domain.
According to this, there are "legal provisions" in place that ensure this.
What exactly are these legal provisions?
While it's hard to prove a negative, it looks to me like the claim is false and arises from a misunderstanding of who pays the levy and who receives it.
For reference, I'm going to cite the relevant passage of your first link from FFUK, the Norwegian fund for performing artists:
Changes in legislation entail that users must pay remuneration to Gramo (joint collecting society in Norway for musicians, performing artists and phonogram producers) for the use of copyright-protected recordings, and a levy to the Fund for the use of non-protected recordings. This applies both to broadcasting and to other public use.
The basis for this appears to come from § 45b of the Copyright Act*:
Sound fixations of the performances of performing artists can, within the period of time specified in section 45, against remuneration, be made available to the pubic [sic] through public performance. [...] Both the producer of the fixation and the performing artists whose performances are reproduced are entitled to remuneration. [...] For public performance of sound fixations that are not protected under the copyright act, the Act No. 4 of 14 December 1956 relating to a levy on the public performance of performing artists’ performances, etc. applies.
FFUK is a fund for performing artists, not one that takes from performing artists. In this situation, it is not the performing artist that pays the levy, but rather those who make use of a "sound fixation" (recording) of a performance of the performing artist.
An example would be a TV network that wishes to broadcast a pre-recorded orchestra performance. If this performance is a song under copyright, then that TV network must pay remuneration to Gramo so that the songwriters, producers, performers etc. can receive compensation.
However, if that song is in the public domain then Gramo has no claim to any remuneration. This is where FFUK is involved. The Act No 4. of 14 December 1956 authorizes levies for these performances of public domain songs, and it is under this act that FFUK operates. The TV network must pay them a levy so that the performers may still be compensated as they still have rights over the performance of a public domain song.
While all this explains the first link you cite, I realize that I'm not directly addressing the claims in the second link. I simply can't find a legal basis for those claims, and still think it's more likely that it's confusing levies for the performance of public domain works (non-existant as far as I can tell) with levies for using performances of public domain works. I would be happy to be corrected if I'm wrong though.
*Linked are unofficial translations of the law because I do not speak Norwegian. The linked Copyright Act is a consolidated version from 2007, the most recent for which WIPO had an English translation. There was a small amendment in 2009, which with the help of Google Translate, does not affect the analysis. There were more extensive amendments in 2015, but those only came into force October 1, 2015, after the date of your question.
