Sounds like the transmission went bad. If it had been setting around without any fluid in it the seals most likely dried out and when new fluid and pressure was applied it finished them off. You could carefully put the pan off and look for seal debris in the fluid.

The seals were replaced though, would you still suggest that ?

So your saying the transmission was overhauled first, then you put it in? If so then they messed something up internally and it most likely will need redone. But if this was a used transmission with a warranty then you need another transmission. I wouldn’t even bother pulling the pan and looking.