Can you PLEASE give me a hand here!

I'm relatively new in VBA programming.
I use a combobox which uses an SQL qry string (strSQL) as a RowSource to select fields from various tables. The strSQL is itself selected according to the value of a option group with values 1, 2 and 3 using "Select Case .." . The first two choices (1 & 2) populate the combobox with numbers from a certain table. Choice no 3 populates it with strings that look something like "1/2/33" (not dates) from another table.
While the programm works fine with the numbers (options 1 & 2) I keep getting the above message every time I try to select something from option 3.
There's no incompatibility in data types - as far as I can tell - and the InputMask and Format properties are left blank.
Funny thing is that the programm worked fine until the day before!
Any hint how to overcome this?

Thanks in advance!