Thank you for the reply, Jesusr.
I fixed the [], and it made no difference. I added the bandname, and it made no difference. The error in LR_Class was flagged at statement 'ParValue := FormatValue(CurValue, AFormat, AFormatStr);' so I set the Format back from Number/Custom/€#,##0.00;(€#,##0.00) to Text/None, and it runs without error. Setting the format to Number/1234,5 again produces the error.
As it works in preview but not at runtime, I presume this is a bug. Any suggestions on a work-around?
Talking of bugs, I note that, in designer, if you try to edit a custom format, the format box is empty, even though the value previously set does show in object inspector.