Is there any plot-wise reason why I am not able to invest into OfS second time? I have some free ProN (500k) at the start of Erosia war and I would like to increase power of OfS, but I don't have an opportunity, which I find rather weird...
I don't know that anyone has asked the dev about it, but I suspect that at the start of the war, it is too late for them to use the money to build themselves into a force that can help more with the war.
Decanter , well, that reason would be legitimate, but you actually CAN invest in OfS before Erosian war, but only if you haven't invested before. And Bartholomew actually says that the order is not ready for work yet. Which means that they are less prepared for war, but if you invest into them, they will give more support than if you invested less into them before, but completed their tasks (killing slimes etc.).
It is kind of weird that you can invest during the ardan succession crisis but only if you haven't invested in the past though, but I think part of the reason is you're kind of infamous by that point and they don't want to accept aid from the doom king's envoys unless they're desperate for it.
Bartholomeuw seems pretty chill about it though, even saying he would give Simon the knighthood if not for the political situation.