Hello @Rodrik I hope you are having a wonderful day!
In order to protect our systems against any malicious users we implemented a ceiling of 1000 projects that a user can publish to workspaces they own. It seems you have reached that limit and are attempting to publish a 1001 project to a workspace you own/have created. Thats a lot of projects, its great to see that you are getting this much value out of RP, as a product manager at Axure this makes me happy!
As a simple workaround I recommend transferring ownership of the Workspace (WS) that you own and are attempting to publish to to instead be owned by someone else who publishes less frequently then you. This will make it so that you can continue to publish to that WS since you will no longer be the owner of the WS and therefor wont run into that 1000 project limit.
Here is a document that outlines the simple steps to transfer WS ownership - https://docs.axure.com/axure-cloud/reference/workspaces/#transferring-workspace-ownership
Hope this helps! If you continue to run into any issues please email into our support team directly at support@axure.com and they will be happy to assist you!
All the best,
Raleigh Matern
Product Manager @ Axure