timekpr not counting correctly

Allowed time per day is 60 minutes, nevertheless user is logged out after 34 minutes. timekpr.log shows that the program should sleep for 30 seconds, sometimes it wakes up after 2 seconds and adds 30 seconds (e.g.14:08:30). Any ideas? Thanks for your help!
Best regards
M
(kubuntu 18.04 kernel 4.15.0-45-lowlatency #48-Ubuntu SMP PREEMPT)
timekpr.log:
2019-02-01 14:06:57 Starting timekpr version 0.3.7
2019-02-01 14:06:57 Starting timekpr version 0.3.7
2019-02-01 14:06:57 Variables: GRACEPERIOD: 120 POLLTIME: 30 DEBUGME: True LOCKLASTS: 30 minutes
2019-02-01 14:06:57 Directories: LOGFILE: /var/log/timekpr.log TIMEKPRDIR: /etc/timekpr TIMEKPRWORK: /var/lib/timekpr TIMEKPRSHARED: /usr/share/timekpr
2019-02-01 14:06:57 Variables: GRACEPERIOD: 120 POLLTIME: 30 DEBUGME: True LOCKLASTS: 30 minutes
2019-02-01 14:06:57 checklockacct called
2019-02-01 14:06:57 Directories: LOGFILE: /var/log/timekpr.log TIMEKPRDIR: /etc/timekpr TIMEKPRWORK: /var/lib/timekpr TIMEKPRSHARED: /usr/share/timekpr
2019-02-01 14:06:57 checklockacct called
2019-02-01 14:06:57 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:06:57 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:07:27 checklockacct called
2019-02-01 14:07:27 checklockacct called
2019-02-01 14:07:27 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:07:27 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:07:57 checklockacct called
2019-02-01 14:07:57 checklockacct called
2019-02-01 14:07:57 configuration file for tobias exists
2019-02-01 14:07:57 configuration file for tobias exists
2019-02-01 14:07:57 User: tobias Day-Index: 5 Seconds-passed: 30
2019-02-01 14:07:57 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:07:57 User: tobias Day-Index: 5 Seconds-passed: 30
2019-02-01 14:07:57 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:08:27 checklockacct called
2019-02-01 14:08:27 checklockacct called
2019-02-01 14:08:27 configuration file for tobias exists
2019-02-01 14:08:27 This day's tobias.time file exists, adding time
2019-02-01 14:08:27 configuration file for tobias exists
2019-02-01 14:08:27 This day's tobias.time file exists, adding time
2019-02-01 14:08:28 User: tobias Day-Index: 5 Seconds-passed: 60
2019-02-01 14:08:28 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:08:30 User: tobias Day-Index: 5 Seconds-passed: 90
2019-02-01 14:08:30 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:08:58 checklockacct called
2019-02-01 14:08:58 configuration file for tobias exists
2019-02-01 14:08:58 This day's tobias.time file exists, adding time
2019-02-01 14:08:58 User: tobias Day-Index: 5 Seconds-passed: 120
2019-02-01 14:08:58 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:09:00 checklockacct called
2019-02-01 14:09:00 configuration file for tobias exists
2019-02-01 14:09:00 This day's tobias.time file exists, adding time
2019-02-01 14:09:00 User: tobias Day-Index: 5 Seconds-passed: 150
2019-02-01 14:09:00 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:09:28 checklockacct called
2019-02-01 14:09:28 configuration file for tobias exists
2019-02-01 14:09:28 This day's tobias.time file exists, adding time
2019-02-01 14:09:28 User: tobias Day-Index: 5 Seconds-passed: 180
2019-02-01 14:09:28 Finished checking all users, sleeping for 30 seconds
2019-02-01 14:09:30 checklockacct called
2019-02-01 14:09:30 configuration file for tobias exists
2019-02-01 14:09:30 This day's tobias.time file exists, adding time
2019-02-01 14:09:30 User: tobias Day-Index: 5 Seconds-passed: 210
2019-02-01 14:09:30 Finished checking all users, sleeping for 30 seconds