Steps to reproduce:

1. I have no clue. I first noticed the issue after I setup a zabbix proxy to monitor hosts which were formerly monitored via VPN.

Result:
Latest Data of the proxies which are monitored by themselves works, and gets updated properly, but shows the read exclamation mark with a message of just a single '. This is also visible in highest debug level, as you can see in the following example:

{{ 15019:20200526:085009.596 itemid:33227 hostid:10307 key:'agent.ping'}}
{{ 15019:20200526:085009.596 type:0 value_type:3}}
{{ 15019:20200526:085009.596 interfaceid:30}}
{{ 15019:20200526:085009.596 state:0 error:'''}}
{{ 15019:20200526:085009.596 flags:0 status:0}}
{{ 15019:20200526:085009.596 valuemapid:10}}
{{ 15019:20200526:085009.596 lastlogsize:0 mtime:0}}
{{ 15019:20200526:085009.596 delay:'1m' nextcheck:1590475847 lastclock:1590475787}}
{{ 15019:20200526:085009.596 data_expected_from:1590475748}}
{{ 15019:20200526:085009.596 history:1 history_sec:604800}}
{{ 15019:20200526:085009.596 poller_type:255 location:0}}
{{ 15019:20200526:085009.596 inventory_link:0}}
{{ 15019:20200526:085009.596 priority:1 schedulable:1}}
{{ 15019:20200526:085009.596 units:'' trends:1}}

It is also reflected by the database:

MariaDB [zabbix]> SELECT * FROM item_rtdata WHERE itemid = 33227;
+--------+-------------+-------+-------+-------+
| itemid | lastlogsize | state | mtime | error |
+--------+-------------+-------+-------+-------+
| 33227  | 0           | 0     | 0     | '     |
+--------+-------------+-------+-------+-------+
1 row in set (0.001 sec)

I have two hosts that are affected, and both of them are active Zabbix Proxies sqlite3 behind NAT with Agent2 monitoring themselves, and sending the data to the Zabbix server.