This is a change in biber 2.1 with
\i in particular. Now biber properly encodes this as a dotless i (ı - 0x0131) with a combining accent. Even though biber always converts to precomposed (NFC) form on output and is therefore generally as friendly as possible to inputenc, there is no precomposed form of this combination. There is a precomposed form for a normal 'i' followed by the same combining char but this is incorrect and it is a completely different thing, causing problems in some fonts and needing a special case in the decoding.
\'i is generally a better choice because most fonts know that the "i" in this shouldn't have both a dot and an accent (or alternatively you simply can't see the dot because of the accent if you are lucky). inputenc also knows about normal ISO8859-1 literal
í characters so you can just use these without any macros.
If you get your data from some external source and therefore don't get to choose the form in which you get your accented "i"s, put in a biber sourcemap to fix up the code if you have inputenc or font issues with dotless "i"s:
Since macro decoding into UTF8 is done before source mapping, it's already in UTF-8 NFD form by the time mappings are applied. The above simply changes the dotless i (U+0131) followed by combining acute accent (U+0301) into a standard ISO 8859-1 lower case i with acute (U+00ED) which is supported by inputenc.
Or you can just use the biber
--output-safechars option to force encoding of UTF-8 into LaTeX macros when writing the .bbl.