It could be reduced water flow or potentially an air bubble causing reduced flow. if the system works fine when set at 40degC, there may be insufficient heat transfer at the elevated temperature. There should be a minimum deltaT of 5degC. 55degc at right at the top end of the capability of the unit. Possibly the heat loss of the system and building prevents the system from achieving 55degc and the unit may be cutting out on high compressor temperature. This would not necessarily indicate a fault code as the system would eventually stop and then restart after a time period.