Declaration of income for 2015.

Good afternoon. We are citizens of Lithuania (European Union). My husband is a sailor. From 2011 to June 2016 he worked on ships from TOS (crewing agency in Holland). TOS paid him a salary after all tax deductions according to Dutch laws. In Lithuania, the husband declared his income every year to the Lithuanian tax office, showing the annual report from the TOS and the taxes paid by the employer. Medical insurance was partially paid by TOS (when the husband was at sea), and when he was on shore, the husband paid medical insurance in Lithuania himself. Never had any problems. In 2017, we received a letter from the Dutch tax office demanding that we pay tax for 2015. The husband turned to TOS since they paid taxes. TOS promised to sort it out. Then they advised my husband to fill out a declaration and send it to the specified address. The letter from the tax office indicated a website where the 2015 declaration could be found. The husband does not know Dutch, but he found the “Personal income statement 2015. Eligible foreign tax” form in English, filled it out, certified it to the Lithuanian tax office and sent it by mail (paper version) to the specified address. The letter was registered, there is a receipt. Two days ago we received a letter from the Dutch tax office demanding that we pay the tax and correct the declaration. My husband wrote to TOS again, but there is no answer yet. Maybe we filled out the wrong form? How to fill out the declaration and who should pay taxes in this situation, because TOS calculated everything from my husband. We have the annual report from TOS and all payslips for each working month of 2015.