What type of report is it? The (correct) runtime for that report must be installed on the Webserver.

There might also be other prerequisites missing:

• Prerequisites IIS
• Web GUI: Set up IIS

Hey Henk-Jan, 
report is devexpress,  initially  we also send the report this works fine, no we only make the pdf and this result in the error

Can you share the setup of your TSFreportmailer task and its parameters and values

These are the parameters in the process flow, but what about those of the task itself?

Can you try setting all the e-mail related parameters to the proper settings when using the non-mailing version of the task?

settings such as

• delete_file
• smtp_only
• smtp_use_ssl

Thanks Henk-Jan

Found it

Points out some of the tasks in the SF of type "GUI code” were configured as Await result = "Yes” which causes the task caller to use an interface which is able to send progress messages back to the shown progress dialog in the client.

Our TSFReportMailer and most of the customer custom written tasks in C# are not ready yet to do so, resulting in this error message.
 

We put Await result to "Yes (without progress)”, which shows only a wait cursor while executing.