Hi Jerry,
the thing is that C. crucibuliforme is older as a name; Scopoli described it in 1772 and Bulliard described Nidularia laevis in 1790. However, Crucibulum laeve is the name that has been used, and because of that there may be around opinions that such a name should be sanctioned because of its wide use. There are lots of such cases where the fungi community is reluctant to give up a name that everyone is using... ;)

OK I got a response which I also need to ty and disenatngle to best of my ability.
My assumption of sanctioning author was incorrect - I thought it couldn't be right. I didn't look hard enough for more sanctioned epithets.

As I understand it the story goes like this ...

The name Peziza crucibulum was sanctioned by Fries in SM2(1):p112, 1822 and he later referred to the same name in the form P. crucibuliformis SM2(2):p610. Apparently he was sloppy in the precise formulation of his epithets and he was writing about the same fungus in both entries. It seems this is the sanctioned entry for Scopoli's Peziza crucibuliformis 1722, ie. Scopoli's name is the earlier homonym of the Fries sanctioned Peziza crucibulum/crucibuliformis. Therefore Scopoli's name is 'unavailable'. However Fries interpretation (redescription) of the fungus in the sanctioning work was different to Scopoli. Fries' name is currently of uncertain application. Meanwhile the fact that Scopoli's name is 'unavailable' means it is only unavailable for Fries' described taxon but is still a legitimate name and can be the basis of a new combination. So Crucibulum crucibuliforme (Scop.) V.S.White is a legitimate name when applied to Scopoli's original description/concept of the taxon.

Meanwhile Persoon in 1801 published Cyathus crucibuliforme which is again a sanctioned name (which is what originally confused me). That is based on Schaeffer 1800 concept of Peziza crucibuliformis. Shaeffer referred to Scopoli's taxon (in the later text, not the original plate), but SpeciesFungorum (SF) implies Schaeffer's concept is a misapplication of Scopoli's name. i.e. this fungus is not considered to be the same as the Scopoli taxon, just like Fries' version isn't. But of course Persoon's sanctioned name cannot be combined in Crucibulum because of the earlier Scopoli name (I think), and/or tautonomy depending on choice of epithet, and thus the correct name for Persoon's taxon is the synonym C. laeve (Hudson) Kambly.

So I think the debate distills down to whether Persoon's fungus is the same Scopoli's from a taxonomic perspective. SF indicates it is not, which is why the two names are not placed in synonymy and Schaeffer's name flagged as a misapplication. If they are considered the same fungus (like Mycobank assumes) then C. crucibularum would be the correct name. If they are not the same then C. crucibularum (Scop) V.S. White (sensu stricto) remains a name of uncertain application and the currently familiar taxon should have the correct name C. laeve. The fact that people have used C. crucibularum intending to refer to the same taxon as C. laeve would be secondary and irrelevant to the nomenclatural issues. Its use would be a misapplication.

So now I am wondering what is the basis of decision that somebody made way back that the Persoon/Schaeffer fungus is not the same as Scopoli's, or has the commonly cited (and nomenclaturally incorrect) synonymy come about simply because people have become confused by the two sanctioned epithets referring to different fungi.

Crystal clear eh?

Any clearer thoughts anybody?

@cooperj : thanks for the in-depth inquiry! I didn't think it would be so complicated... I have no personal advice on that matter. The most important is to recognize that both taxa are synonyms I guess. On iNat C.laeve has much more observations than C.crucibuliforme. So this is an additional argument in favor of the widely used name...