Even with the Brother own drivers installed, the scan part of "Brother DCP L2550DN" is not working (scan not found, sometimes even scanning but returning an error) on simple-scan or xsane in Ubuntu 20.04, while it was working on earlier versions.
(Note that this is a solved question, posted for others to get the solution quickly)
Brother released a "patch" deb specific for Ubuntu 20.04 (and derivatives) that solves the problem (possibly related to this bug):
https://support.brother.com/g/b/faqend.aspx?c=eu_ot&lang=en&prod=hll6300dw_eu&ftype3=100257&faqid=faq00100729_000
Note: While personally tested on DCP L2550DN, this solution should also apply to other Brother multifunction printer/scanners, like e.g. HL-L6300DW(T)