Update Entity error - Participating entity record of stage is not valid

Hi all,

i'm having an issue in trying to Update an Entity, in particular, i want to update the active stage of an Opportunity in the BPF and i'm having the error:

'participating entity record of stage is not valid'

Here's the code i'm using:

newActiveStageId = (Guid)pathResp.ProcessStages.Entities[activeStagePosition].Attributes["processstageid"];

// Retrieve the process instance record to update its active stage
ColumnSet cols1 = new ColumnSet();
cols1.AddColumn("activestageid");

Entity retrievedProcessInstance = crmSvc.Retrieve("opportunitysalesprocess", processOpp1Id, cols1);

// Update the active stage to the next stage
retrievedProcessInstance["activestageid"] = new EntityReference("processstage", newActiveStageId);
crmSvc.Update(retrievedProcessInstance);

crmSvc.Update(...); is the line affected by the error.

I've not found any usefull suggestion about this kind of error, so if someone can help it would be great.

Thanks for anyone responding.