In the following, I will refer what I, as a layman, have drawn from Olive Sayce (1967), "Carmina Burana 180 and the mandaliet refrain", in: Oxford German Studies, vol. 2, p. 1-12.
The refrain in question, from Carmina Burana 180, reads:
Manda liet manda liet min geselle chǒmet niet.
It is attached to a Latin poem, which shares the love letter theme with the preceding German one, 179a, Einen brief ich sande / einer vrowen gůt. However, the content of the German refrain of 180 has nothing to do with the Latin poem and "it can only have been fortuitously attached to it" (Sayce; she also notes that the surrounding poems all have refrains, which might have been a reason for attaching one to 180).
The mandaliet refrain resembles that of 180a:
Ich sage dir ih sage dir min geselle chum mit mir.
Now, with regard to the meaning of the mystery word mandaliet, Sayce points out that although menden "rejoice" did exist (see AWB), it was archaic at the time, and no compounds with manda- are attested.
If the word mandaliet doesn't exist, what could it mean? Well, nothing, says Sayce:
It is certainly an invented nonce-word of the type very common in refrains. It is simply there to provide (a) the requisite number of beats demanded by the metre, (b) the rhyme. That the second element should take the form of a real word for the sake of the rhyme, though meaningless in the context in which it occurs, is also a device of common occurrence.
Sayce also argues that the refrain was taken from a dance song (which I find quite plausible; after the solo part, the crowd would join in on the mandaliets). So although Wackernagel's interpretation of mandaliet as "dance-song" (recorded by Lexer) may have been wrong on a linguistic level, he might have been right about what the refrain is conveying.
If you want to check out the manuscript of Carmina Burana 180, take a look at MDZ, or go here for a transcription. You may also enjoy listening to the song "Mandaliet" by the band Dunkelschön, from their album Katharsis (2009).