I hope you’re well.
I’d like to know if it’s possible to use an EB55 (for example plug in my PC) while it is being charged with a solar panel.
Yes, See the excellent review by Moderator Scott Benson.
Yes, you can use it like a solar powered UPS running loads while charging. I have a EB55 myself.
In the photo you can see that it is charging at 80 W while the loads are pulling 267 W. I was testing it here, even with my cellphone on the top.