Question: (in dsolve) reducible RootOf detected.. what is this?

I have define the following ODE system

restart;
with(LinearAlgebra);
dC1 := diff(C1(t), t) = -2*a1*C1(t)+b1*C2(t);
dC2 := diff(C2(t), t) = 2*a1*C1(t)-(b1+a1)*C2(t)+2*b1*C3(t);
dC3 := diff(C3(t), t) = a1*C2(t)-(2*b1+a2)*C2(t)+b2*O1(t);
dO1 := diff(O1(t), t) = a2*C3(t)-b2*O1(t);
syst := dC1, dC2, dC3, dO1;ics := C1(0) = 1, C2(0) = 0, C3(0) = 0, O1(0) = 0;
func := {C1(t), C2(t), C3(t), O1(t)};
sol := dsolve([syst, ics], func);

After evaluating the system with dsolve I obtain a message that start by saying: "Error, (in dsolve) reducible RootOf detected..."

How do I solve my system and obtain the solutions for my functions?

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