"But can you check, whether the Cacti graphs show real traffic?"
Yes, it shows real traffic

"It's an issue, I've actually fixed (so I think) in 1.8. You can try this, to keep the discovery going:"
inc/libmisc.pm (I think you mean this file?) is now:
if($main::int{$dv}{$i}{old}){
my $dval = $val - $main::int{$dv}{$i}{$abs};
if( $val eq '1.84467440737096e+19' ){ # Some Cisco routers got counters stuck at > 64bit!
$val = 1;
Prt("ERR :$abs counter stuck > 64bit!\n",'');
}elsif( $val < 18446744073709551615 and $dval == abs $dval ){ # Nexus can produce overflowing 64-bit counters causing NeDi to die at DB write!
$main::int{$dv}{$i}{$dlt} = $dval;
}else{
Prt("ERR :$abs overflow, not updating\n",'');
But the error occurs anyway...
"I see it in your debug output on interface Bundle-Ether1.196. Is this supposed to be an IPv6 address?"
Yes this intarface has a IPv4 and IPv6 address.
/var/www/nedi/nedi.pl -a r03
Started with 1 seed at Wed Mar 6 15:23:31 2019
-------------------------------------------------------------------------------
Device Status Todo/Done-Time
===============================================================================
192.168.1.1 r03 Iv i50 JxJxJxJxJxJxJxJxJxJxJxJxUse of uninitialized value in concatenation (.) or string at /var/www/nedi/inc/libsnmp.pm line 1915.
j48 Dcp0 b0 m10 Ap a0/0 f0 0/1-5s
===============================================================================
1 devices, 3 threads, retire: 0 MAC links in 1 iterations, 0 seconds
Took 0 minutes
/var/www/nedi/nedi.pl -a r01
Started with 1 seed at Wed Mar 6 15:24:25 2019
-------------------------------------------------------------------------------
Device Status Todo/Done-Time
===============================================================================
192.168.2.2 ham-apc-r01 Iv i112 JxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxJxUse of uninitialized value in concatenation (.) or string at /var/www/nedi/inc/libsnmp.pm line 1915.
j143 p0 b0 m8 Ap a0/0 f0Ru(MgmtEth0%2FRSP0%2FCPU0%2F1)Ri(Bundle-Ether1.120)Ri(Bundle-Ether1.147)Ri(Bundle-Ether1.198)Ri(Bundle-Ether1.121)Ri(Bundle-Ether1.105)Ri(Bundle-Ether1.510)Ri(Bundle-Ether1.171)Ri(Bundle-Ether1.161)Ri(Bundle-Ether1.156)Ri(Bundle-Ether1.164)Ri(Bundle-Ether1.117)Ri(Bundle-Ether1.229)Ri(Bundle-Ether1.188)Ri(Bundle-Ether1.226)Ri(Bundle-Ether1.131)Ri(Bundle-Ether1.134)Ri(Bundle-Ether1.124)Ri(Bundle-Ether1.2)Ri(TenGigE0%2F0%2F2%2F0)Ri(Bundle-Ether1.207)Ri(Bundle-Ether1.218)Ri(Bundle-Ether1.208)Ri(dwdm0%2F0%2F2%2F3)Ri(Bundle-Ether1.215)Ri(Bundle-Ether1.230)Ri(Bundle-Ether1.531)Ri(dwdm0%2F0%2F2%2F0)Ri(Bundle-Ether1.122)Ri(Bundle-Ether1.144)Ri(Bundle-Ether1.153)Ri(Bundle-Ether1.148)Ri(Bundle-Ether1.9)Ri(Bundle-Ether1.193)Ri(Bundle-Ether1.174)Ri(Bundle-Ether1.194)Ri(MgmtEth0%2FRSP0%2FCPU0%2F0)Ri(Bundle-Ether1.108)Ri(Bundle-Ether1.245)Ri(Bundle-Ether1.167)Ri(dwdm0%2F0%2F2%2F1)Ri(Bundle-Ether1.203)Ri(Bundle-Ether1.149)Ri(TenGigE0%2F0%2F2%2F1)Ri(Bundle-Ether1.200)Ri(Bundle-Ether1.115)Ri(Bundle-Ether1.172)Ri(Bundle-Ether1.136)Ri(Bundle-Ether1.176)Ri(Bundle-Ether1.197)Ri(Bundle-Ether1.219)Ri(Bundle-Ether1.4)Ri(Bundle-Ether1.231)Ri(Bundle-Ether1.19)Ri(Bundle-Ether1.242)Ri(Bundle-Ether1.102)Ri(Bundle-Ether1.175)Ri(dwdm0%2F0%2F2%2F2)Ri(Bundle-Ether1.125)Ri(Bundle-Ether1.204)Ri(Bundle-Ether1.196)Ri(Bundle-Ether1.165)Ri(Bundle-Ether1.116)Ri(Bundle-Ether1.118)Ri(Bundle-Ether1.158)Ri(Bundle-Ether1.163)Ri(Bundle-Ether1.169)Ri(Bundle-Ether1.154)Ri(Bundle-Ether1.142)Ri(Bundle-Ether1.202)Ri(Bundle-Ether1.221)Ri(Bundle-Ether1)Ri(Bundle-Ether1.238)Ri(Bundle-Ether1.213)Ri(Bundle-Ether1.145)Ri(Bundle-Ether1.243)Ri(Bundle-Ether1.110)Ri(Bundle-Ether1.152)Ri(Bundle-Ether1.107)Ri(Bundle-Ether1.157)Ri(Bundle-Ether1.212)Ri(Bundle-Ether1.184)Ri(Null0)Ri(Loopback0)Ri(Bundle-Ether1.507)Ri(Bundle-Ether1.179)Ri(Bundle-Ether1.236)Ri(Bundle-Ether1.216)Ri(TenGigE0%2F0%2F2%2F3)Ri(Bundle-Ether1.214)Ri(Bundle-Ether1.14)Ri(Bundle-Ether1.237)Ri(Bundle-Ether1.240)Ri(Bundle-Ether1.50)Ri(Bundle-Ether1.151)Ri(Bundle-Ether1.241)Ri(Bundle-Ether1.103)Ri(Bundle-Ether1.139)Ri(Bundle-Ether1.205)Ri(Bundle-Ether1.246)Ri(Bundle-Ether1.201)Ri(Bundle-Ether1.206)Ri(Bundle-Ether1.183)Ri(Bundle-Ether1.178)Ri(Bundle-Ether1.109)Ri(Bundle-Ether1.217)Ri(Bundle-Ether1.189)DBD::mysql::st execute failed: Out of range value for column 'outoct' at row 1 at /var/www/nedi/inc/libdb.pm line 1476.
DBD::mysql::st execute failed: Out of range value for column 'outoct' at row 1 at /var/www/nedi/inc/libdb.pm line 1476.