Error: "There was a problem retrieving the task. This task is in an invalid state." when trying to complete a task

When user (even with Admin account) trying to complete a task or when admin trying to 'view form' from the process modeler, the message "There was a problem retrieving the task. This task is in an invalid state." appears. I tried to skip the problematic node but still getting the same error on the following one. Tried restarting the sub-process but same error happens.

Does enayone know where this error is coming from and /or how to solve it?

Thanks

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