Sensu WebUI offline - restarting service solves the problem

Sure.

sensu-backend ~ » sudo cat /etc/sensu/backend.yml
---
state-dir: "/var/lib/sensu/sensu-backend"
api-url: https://sensu-backend.example.org:8080
cert-file: "/etc/sensu/ssl/cert.pem"
key-file: "/etc/sensu/ssl/key.pem"
trusted-ca-file: "/etc/sensu/ssl/ca.crt"

If it helps I also can grant access to this VM.

Additional info about the system:

sensu-backend ~ » sudo cat /proc/cpuinfo                                                                                                                                                                                                                   2 ↵
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 85
model name	: Intel Xeon Processor (Skylake, IBRS)
stepping	: 4
microcode	: 0x1
cpu MHz		: 2294.608
cache size	: 16384 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology cpuid tsc_known_freq pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch cpuid_fault invpcid_single pti ssbd ibrs ibpb fsgsbase bmi1 hle avx2 smep bmi2 erms invpcid rtm avx512f avx512dq rdseed adx smap clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 arat pku ospke md_clear
bugs		: cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs taa itlb_multihit
bogomips	: 4589.21
clflush size	: 64
cache_alignment	: 64
address sizes	: 40 bits physical, 48 bits virtual
power management:

processor	: 1
vendor_id	: GenuineIntel
cpu family	: 6
model		: 85
model name	: Intel Xeon Processor (Skylake, IBRS)
stepping	: 4
microcode	: 0x1
cpu MHz		: 2294.608
cache size	: 16384 KB
physical id	: 0
siblings	: 2
core id		: 1
cpu cores	: 2
apicid		: 1
initial apicid	: 1
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology cpuid tsc_known_freq pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch cpuid_fault invpcid_single pti ssbd ibrs ibpb fsgsbase bmi1 hle avx2 smep bmi2 erms invpcid rtm avx512f avx512dq rdseed adx smap clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 arat pku ospke md_clear
bugs		: cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs taa itlb_multihit
bogomips	: 4589.21
clflush size	: 64
cache_alignment	: 64
address sizes	: 40 bits physical, 48 bits virtual
power management:
sensu-backend ~ » free -hm                                                                                                                                                                                                                               130 ↵
              total        used        free      shared  buff/cache   available
Mem:          3,8Gi       260Mi       2,8Gi        24Mi       695Mi       3,3Gi
Swap:            0B          0B          0B