Compile errors on dropbear

Building custom image, I get lot of compile errors for dropbear.
This is the first one:

/etc/lede/ZBT826/staging_dir/toolchain-mipsel_24kc_gcc-7.4.0_musl/lib/gcc/mipsel-openwrt-linux-musl/7.4.0/../../../../mipsel-openwrt-linux-musl/bin/ld: /etc/lede/ZBT826/tmp/ccxp20NO.ltrans0.ltrans.o: in function dropbear_big_endian_ctr_start': <artificial>:(.text+0x988): undefined reference toctr_start'

Any idea, how to solve it ?

Have you already tried a make dirclean?

Actually, I suspect some unknown "speciality" of Centos to be the problem. As the same build works on ubuntu.