Thanks L&LD and SheikhSheikha!
I will go through that in the evening after work, since I am currently working over a tethered hotspot of my mobile phone. Since flashing the router (I did a firmware reset before flashing) with Merlin, all of my devices get constantly dropped of the wifi, so there is no chance to work
But I also didn't see/notice L&LD's instructions...
Jun 22 10:32:40 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:32:41 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:32:46 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 7C:XX:XX:XX:XX:XX, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:32:46 syslog: WLCEVENTD wlceventd_proc_event(401): wl0.1: Disassoc 7C:XX:XX:XX:XX:XX, status: 0, reason: Disassociated because sending station is leaving (or has left) BSS (8)
Jun 22 10:32:48 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:32:48 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:32:57 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 7C:XX:XX:XX:XX:XX, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:33:03 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 90:YY:YY:YY:YY:YY, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:03 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 90:YY:YY:YY:YY:YY, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:09 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:09 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:09 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:12 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 90:YY:YY:YY:YY:YY, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:33:15 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 7C:XX:XX:XX:XX:XX, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:33:15 syslog: WLCEVENTD wlceventd_proc_event(401): wl0.1: Disassoc 7C:XX:XX:XX:XX:XX, status: 0, reason: Disassociated because sending station is leaving (or has left) BSS (8)
Jun 22 10:33:17 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:17 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:23 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 90:YY:YY:YY:YY:YY, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:23 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 90:YY:YY:YY:YY:YY, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:23 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 90:YY:YY:YY:YY:YY, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:26 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 7C:XX:XX:XX:XX:XX, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:33:33 syslog: WLCEVENTD wlceventd_proc_event(386): wl0.1: Deauth_ind 90:YY:YY:YY:YY:YY, status: 0, reason: Deauthenticated because sending station is leaving (or has left) IBSS or ESS (3)
Jun 22 10:33:33 syslog: WLCEVENTD wlceventd_proc_event(401): wl0.1: Disassoc 90:YY:YY:YY:YY:YY, status: 0, reason: Disassociated because sending station is leaving (or has left) BSS (8)
Jun 22 10:33:38 syslog: WLCEVENTD wlceventd_proc_event(420): wl0.1: Auth 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
Jun 22 10:33:38 syslog: WLCEVENTD wlceventd_proc_event(449): wl0.1: Assoc 7C:XX:XX:XX:XX:XX, status: 0, reason: d11 RC reserved (0)
And that's just a few lines of the whole 2.5k log