I only have 1 device with this program, what is going on
Could you confirm which ProtonVPN subscription you have? Free plans can have 1 VPN connection, whereas paid plans offer more simultaneous VPN connections.
“Protonvpn free only allows 1 device. Once you are connected with one device, you can’t connect with any more devices.”
I kept having this issue a while back. Assuming you’re on Windows, kill the program via the task manager and restart it. That always fixed it for me.
If you have an enterprise version you need to allocate the number of connection to each email account. Weird enough these settings are in the email account settings at the same spot were you can create email accounts and aliases. You have to edit the email account. There you can allocate the number of connections allowed.
One device is the max 
Only use it on that device
Same here, I guess it’s thinking you are still connected. I wish they offered the option to disconnect all other connections and use this one. I forget what subscription based program I used to use did this.
Why would I think my desktop isn’t a device?
It sometimes happens when your IP address gets changed.
For example on your mobile phone, when you switch from “Mobile data like LTE/3G/4G” to “WiFi”, you are connecting from another IP address even though using same device. In this situation you will have to wait for like a couple of minutes for previous session to be expired OR in order to prevent that problem happening, you can hit the disconnect button and wait for connection to be disconnected before disconnecting your device from network (Wifi, 4g …)and connecting to another network. Basically telling the servers that hey I’m leaving, so you will not have to wait 1 or 2 minutes for servers finding out that you just left.
And it could also happened if you didn’t changed your network but you ISP decided to change your IP and somehow it could happen. I’m not a expert in this but in my country, it could happen.
Alternatively purchasing the paid plan would fix this.
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By the way, it is only my personal experience and I’m just a ProtonVPN user and maybe I’m wrong.
Peace 
Can confirm that this has happened to me before as well. Usually for me it happens when I connect/disconnect to different servers in quick succession. But yeah, waiting for a few minutes before trying again always solves the problem
Yeah, that’s a different story but same problem. Waiting for a minute will always fix that.