  The complete nonperturbative expressions for the high-temperature expansion
of the one-loop effective action induced by the charged scalar and the charged
Dirac particles both at zero and finite temperatures are derived with account
for possible nontrivial boundary conditions. The background electromagnetic
field is assumed to be stationary and such that the corresponding Klein-Gordon
operator or the Dirac Hamiltonian are self-adjoint. The contributions of
particles and antiparticles are obtained separately. The explicit expressions
for the $C$-symmetric and the non $C$-symmetric vacuum energies of the Dirac
fermions are derived. The leading corrections to the high-temperature expansion
due to the nontrivial boundary conditions are explicitly found. The corrections
to the logarithmic divergence of the effective action that come from the
boundary conditions are derived. The high-temperature expansion of the naive
one-loop effective action induced by charged fermions turns out to be divergent
in the limit of a zero fermion mass.
