Thanks for getting back with your reply.
I installed Scheduler for the first time according to Scheduler Setup recommendation.
As it is first time installation it did not work before. But according to my testing I might not be problem of the Scheduler itself. It behaves the same when I run the workflow from the Architect manually.
When the scheduler service is up and ruunning the workqueue processing is turned on for the Origam server instance in OrigamSettings.config by <AutoProcessWorkQueues>true</AutoProcessWorkQueues>
Actually the process works the way Scheduler only processes 1 workflow e.g. Transakce that creates rows in WQ_Transakce. Then WQ_Transakce is autoproccessed (correctly btw) and generates all email parameters that are passed to WQ_Email of Mail WorkQueueClass that has the only purpose to process all records and send emails by command Send.
WQ_Transakce is always proccessed correctly and thus I assume, Scheduler works just fine. The error happens when WQ_Email is proccessed.
Sorry for long answer, but this description reveals the behaviour in deatail and probably explains more where the error happens.