This might be helpful for other people.
I decided to give the PTS a try this afternoon. So I clicked on the "install" button on the PTS server side of my launcher. Turns out, there wasn't enouugh space on my drive to install the beta version. Well, okay, I thought.
Then I came back to the game this evening. The PTS wasn't mentioned anymore, but the Install button had replaced the play button on the PC/EU side of the launcher. I clicked on it, and again, error 104, there wasn't enough space to install it. I tried to restart the computer, to turn it on and off again, but I haven't regained my "play" button.
In other words, I had to uninstall several other softwares to make room for the game, which, by the way, is still downloading at the moment, even though I never, ever uninstalled ESO.

Not a very nice bug.
Edited by VerboseQuips on August 1, 2017 6:18PM My characters:
Main and crafter: A Breton magicka templar named Erwann Sorril
Alt 1: A Bosmer sorcerer named Tuuneleg
Alt 2: An Imperial dragonknight named Gaius Tullius Hastifer
Alt 3: An Argonian vampire/nightblade named Observe-le-Xanmeer
Alt 4: A Nord werewolf/dragonknight named Sigurd Hurlevent
Alt 5: A Breton sorcerer named Gilian Sorril (he's Erwann's younger brother)
Alt 6: A Khajiit nightblade named Jolan-dar
Alt 7: A Nord warden named Sigurmar Hurlevent (he's Sigurd's younger brother)
Alt 8: An Altmer templar named Oioriel
Alt 9: An Argonian stamina Warden named Danse-avec-les-Rainettes
Alt 10: A Redguard templar named Neemokh af-Corelanya
Alt 11: A Nord stamina sorcerer named Olga Écoute-Vent
Alt 12: A Breton magicka Warden named Ian Sorril
Alt 13: A Dunmer magicka necromancer named Ilmoran Dren
Alt 14: An Orc stamina necromancer named Norgol gro-Borziel
Alt 15: A Nord magicka necromancer named Thorgen Givresang
Alt 16: An Imperial magicka dragonknight named Publius Valeirus Hastifer (Just call him "Valerio" - he's Gaius younger troublemaker of a brother)
Main in NA (For collaborative events): A Breton magicka nightblade named Titouan Sorril (long-lost brother of Erwann and Gilian)