I have in my cluster 3 nodes (1 master version 9.6.3+ 2 slaves version 9.6.3). I configured repmgr (with repmgrd active) v 4.0.4.
Suddenly today after a few good weeks I noticed that there is a lag in one of the slaves and the error in the log indicated that the slave didnt get the wal :
could not receive data from WAL stream: ERROR: requested WAL segment 0000000900002E61000000BD has already been removed
In which node did you check for the file? If the file in the primary is still available, try to compare their md5sum . If you have a working WAL shipping method in place, then add the appropriate line in the recovery.conf of your standby :