My wife was denied boarding on June 15th 2018 by WizzAir staff at Luton Airport, although we had all the required documents to enter Poland.
She holds a valid UK travel document and a UK resident permit valid for another 2 years. This is what happened at Luton:
1. The airline agent requested that she needs to have a Shengen visa. However, after spending 3 hrs discussing the matter with the Polish boarder agency and the Polish embassy in London, it became clear that she does not need a visa and can enter Poland without a visa. Even the First Secretary of the Polish embassy in London personally send an email to the airline agent Elizabeth Strose confirming that “pursuant to notifications of the Member States to the Article 4(2) of the Council Regulation 539/01, holder of British refugees travel document (issued on the basis of the Convention of 28 July 1951) who hold a residence permit for the UK (in addition to the travel document), is exempt from the visa requirement, provided that his/hers stay in Poland does not exceed 90 days.” This is also mentioned on Poland’s embassy website.
2. After the agent was convinced that there is no need for a visa, she claimed that “Wizz Air does not allow anyone with a travel document even with a visa to travel to Poland”!!!

This has not been mentioned anywhere and I checked with your customer service on the phone that this statement does not reflect Wizz Air’s policy.
3. As a result of such incompetent, we could not get the 5:30 and then 20:00 flights, and after spending 7 hrs on the airport, we had to leave for home. We missed a close friend’s wedding in Poland and lost around £600 (for hotel and car rental) only because the agents were not competent and they were not aware of latest regulations. I believe this would not have happened if we were flying with any other airline.