Hi,
I believe version 11, doesn't provide sub task option by default(correct me if am wrong), so most probably the error will be coming from the custom codes.

So you have to check your log and update the code such a way that. code can handle multiple records.

See:  Singleton Error In Odoo

Thanks

Hi

Maybe it comes in your customized code, the project task has different records in the same project that's why it happened.

you can refer to the blog: https://www.cybrosys.com/blog/how-to-handle-singleton-error-in-odoo-15 it will help to reduce the single-ton issue

Hope it helps