What is the factual basis for Guaido and his supporters' claim that Maduro's election as president is not valid?
It's not that the actual election procedure was problematic, it's that political opponents of Maduro were jailed, or have fled the country. In those circumstances, you could not call the 2018 Venezuelan elections free or fair.
However, this is not an uncommon practice in Latin America, the middle east or so Asia. In Brazil, Lula De Silva, a massively popular pink tide, progressive politician was jailed and prevented from running for president last year too. America, Canada, the EU etc turned a blind eye to this. Based on polls, Lula would have won the 2018 election by a landslide,instead, Bolsonaro was elected, and I hope we're all aware of his record regarding political opponents, indigenous people, the environment, the lgbt community etc. So while Maduro cannot claim to have won free and fair elections, nor can Bolsonaro. The reasons why Venezuela is being targeted for regime change and not Brazil should be obvious to all.
You also have to take into context the long history of American backed coups and regime change war efforts in Latin America, always in support of far right extremists and dictators who are open to privatization of state resources by American multinationals, and the fact that Venezuela has already weathered such coup attempts, even a drone assassination attempt. I'm not saying that this excuses arresting political opponents, but it should be presented as a part of the context at the very least, if we're not to present a disingenuous, one side argument.