welcome to the forum, ewiandr. the problem is that your list contains more than 1 result. so for eg, if mylist contains:
XYZ
ABC

then FIND(mylist,d1) would produce 2 results (this is called an array):
{#VALUE!;1}
XYZ cannot be found, so it returns an error. ABC is found in the 1st character, hence it returns 1. to get the above result, you can select that portion of the formula & press F9 to calculate

so you need to find out if they are numbers. and you did that by doing ISNUMBER(FIND(mylist,d1)). that would produce:
{FALSE;TRUE}

so as long as 1 is true, it should be a "y". i need to convert them into numbers to count them. a single negative makes TRUE into -1, FALSE into 0. 2 negations make it positive. so
--ISNUMBER(FIND(MyList,D1))
that gives me:
{0;1}

i cannot do a simple SUM to add them up. because they are an array. array requires a combination of CTRL + SHIFT + ENTER if you use the SUM. you can avoid that by doing SUMPRODUCT:
SUMPRODUCT(--ISNUMBER(FIND(MyList,D1)))

so as long as SUMPRODUCT is 1, it will input a "y":
=IF(SUMPRODUCT(--ISNUMBER(FIND(MyList,D1))),"Y","N")

also, do note that FIND is case sensitive. so if your list is ABC & D1 is abc123, it won't work. use SEARCH if you want it to be non-case sensitive. another way is:
=IF(ISNA(LOOKUP(2^15,FIND(MyList,D1))),"n","y")