Since nobody is answering:
The reason for the above problem is the fact that apparently the following two packages are obsoleted by EPEL:

Guam logs:
2020-11-07 07:20:13.817 [error] <0.396.0> Supervisor {<0.396.0>,kolab_guam_listener} had child session started with {kolab_guam_session,start_link,undefined} at <0.18081.1> exit with reason no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in context child_terminated
2020-11-07 07:20:13.818 [error] <0.18185.1> gen_server <0.18185.1> terminated with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178
2020-11-07 07:20:13.819 [error] <0.18185.1> CRASH REPORT Process <0.18185.1> with 0 neighbours exited with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in gen_server:terminate/7 line 826
2020-11-07 07:20:13.819 [error] <0.396.0> Supervisor {<0.396.0>,kolab_guam_listener} had child session started with {kolab_guam_session,start_link,undefined} at <0.18185.1> exit with reason no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in context child_terminated
2020-11-07 07:20:13.819 [error] <0.18130.1> gen_server <0.18130.1> terminated with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178
2020-11-07 07:20:13.819 [error] <0.18130.1> CRASH REPORT Process <0.18130.1> with 0 neighbours exited with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in gen_server:terminate/7 line 826
2020-11-07 07:20:13.819 [error] <0.396.0> Supervisor {<0.396.0>,kolab_guam_listener} had child session started with {kolab_guam_session,start_link,undefined} at <0.18130.1> exit with reason no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in context child_terminated
2020-11-07 07:20:13.826 [error] <0.18128.1> gen_server <0.18128.1> terminated with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178
2020-11-07 07:20:13.826 [error] <0.18128.1> CRASH REPORT Process <0.18128.1> with 0 neighbours exited with reason: no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in gen_server:terminate/7 line 826
2020-11-07 07:20:13.826 [error] <0.396.0> Supervisor {<0.396.0>,kolab_guam_listener} had child session started with {kolab_guam_session,start_link,undefined} at <0.18128.1> exit with reason no match of right hand value {error,closed} in kolab_guam_session:accept_client/1 line 178 in context child_terminated

Happens as well on CentOS 7 after installing the current guam update guam-0.9.8-1.3.el7.kolab_16.x86_64