Probably it's not about filling type, because I tried on 3 different mt5. On two I get that "Invalid volume in the request". Mt5 makes some trades, but majority fails. On 3rd mt5 which is ActivTrades I get your error about filling type, and here this fix works. However after changing order type to correct as link suggest I start to get same invalid volume error with couple trades and rest is not made... It happens with about 60% of EAs, and with other it does not. However I can not find out how these EAs differs from each other to find out at what place it starts to spit these errors. EAs seems to be just with different indicators.
My bet it is more a case about bad trade management/lot sizing in created mql5 code, because "invalid volume". But ye, I should be humble here, because I am not programmer just guessing wildly.
I asked my broker about order filling type. They do not know, and said I should ask EA developer about this
Would be nice if someone confirms that their mt5 EAs works flawlessly on tester with "open prices only", or they are getting same errors in Journal. I saw in forum post that other people struggle with mt5. Maybe it is the same case?