Apparently the problem lies within BC and the CodeUnit 1261 itself.
With the help of Mr. Reeps we were able to modify the CodeUnit 1261 so that we could run a import of camt52 data successfully.
Modification of the following Constants in CodeUnit 1261:
StatementIDTxt: Label '/Document/BkToCstmrStmt/Stmt/Id', Locked = true;
IBANTxt: Label '/Document/BkToCstmrAcctRpt/Rpt/Acct/Id/IBAN', Locked = true;
BankIDTxt: Label '/Document/BkToCstmrStmt/Stmt/Acct/Id/Othr/Id', Locked = true;
CurrencyTxt: Label '/Document/BkToCstmrStmt/Stmt/Bal/Amt', Locked = true;
BalTypeTxt: Label '/Document/BkToCstmrAcctRpt/Rpt/Bal/Tp/CdOrPrtry/Cd', Locked = true;
ClosingBalTxt: Label '/Document/BkToCstmrAcctRpt/Rpt/Bal/Amt', Locked = true;
StatementDateTxt: Label '/Document/BkToCstmrAcctRpt/Rpt/CreDtTm', Locked = true;
CrdDbtIndTxt: Label '/Document/BkToCstmrStmt/BkToCstmrAcctRpt/Rpt/CdtDbtInd', Locked = true;