Unexpected error while using AJAX

[FRW]

I have a select_single field with a lookup table containing 9000 rows. I choose "user friendly search" and all is ok - then I choose "use ajax to load" I could save this but get "Unexpected error" at the frontside every time

Setting "use ajax to load" to "NO" everything is still okay

 

[FRW]

I think I've found the error - the corresponding field for the select_single was INTEGER. I changed it to VARCHAR and loading with ajax works fine