If your problem is lack of information:
The offices in national tax administrations of EU countries dealing specifically with cross-border tax issues would be a good first point of contact. They will be able to give you comprehensive information on the tax systems in question.
Europe Direct offers a central information service reachable by phone or internet and local information services in every EU country that provide information tailored to local needs. From Europe Direct you can get an immediate answer to general questions related to the EU or you can obtain directions to the best sources of information/advice and contact details (at EU, national or local level). You can also obtain information on your rights and opportunities as an EU citizen and how to exercise them.
If you need more specialised advice, the Europe Direct Office will refer you to Your Europe Advice service (you can also directly contact that service). This is an EU advice service for the public. The experts cover all EU official languages and are familiar both with EU law and national laws in all EU countries.
EURES provides information about and helps to solve all sorts of problems related to cross-border commuting that workers and employers living and working in different EU countries may experience.
If you think that your rights as an EU citizen are being overlooked or breached or if you are being taxed by two or more countries, there are various sources of help:
The offices in national tax administrations of EU countries dealing specifically with cross-border tax issues would be a good first point of contact. There are often specific procedures to solve different types of cross-border problems, that depend on the two countries involved, and these offices should be able to assist you with that issue.
SOLVIT is an on-line problem solving network provided by EU countries themselves. It aims to solve problems caused by the misapplication of EU Internal Market law by public authorities in one or more countries. There is a SOLVIT centre in every European Union country (as well as in Norway, Iceland and Liechtenstein) and they work together in cross-border cases. SOLVIT Centres can help with handling complaints from both citizens and businesses. They are part of the national administrations and are committed to providing solutions to problems within ten weeks. Using SOLVIT is free of charge.
Ombudsmen in the EU countries can provide assistance in cases where citizens have been a victim of maladministration on the part of the authorities, including tax authorities.
The European Commission can, if an EU country applies rules that appear to be contrary to EU law, launch legal proceedings that could end at the Court of Justice of the EU.
But note that any action taken by the Commission against an EU country may not have an automatic or immediate impact on your individual rights as a complainant. A court judgment merely obliges the EU country to amend its law in line with EU law and may prevent unequal treatment in the future but will not necessarily resolve cases that have already occurred. Please be advised, therefore, that even if you decide to file a complaint with the Commission you should in parallel start or continue to pursue your case with national authorities or courts.