Relaxation of a two-level system (TLS) into a resonant high-temperature bosonic reservoir is considered. The reservoir can be described by a complex Gaussian-Markovian field. This problem can be relevant for atoms in high-Q cavities and for muon spin depolarisation. Analytical solution of the problem in the strong-coupling limit is obtained. The relaxation of the TLS appears to be nonexponential, the coherence, unexpectedly, decaying much slower then the population.