diff --git a/pbs-datastore/src/backup_info.rs b/pbs-datastore/src/backup_info.rs
index 4b222225..d4732fdd 100644
--- a/pbs-datastore/src/backup_info.rs
+++ b/pbs-datastore/src/backup_info.rs
@@ -27,7 +27,7 @@ pub const DATASTORE_LOCKS_DIR: &str = "/run/proxmox-backup/locks";
 // of the file. this should only happen if a user messes with the `/run/proxmox-backup` directory.
 // if that happens, a lot more should fail as we rely on the existence of the directory throughout
 // the code. so just panic with a reasonable message.
-static OLD_LOCKING: LazyLock<bool> = LazyLock::new(|| {
+pub(crate) static OLD_LOCKING: LazyLock<bool> = LazyLock::new(|| {
     std::fs::exists("/run/proxmox-backup/old-locking")
         .expect("cannot read `/run/proxmox-backup`, please check permissions")
 });
diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
index 008449e5..8719c2cd 100644
--- a/pbs-datastore/src/datastore.rs
+++ b/pbs-datastore/src/datastore.rs
@@ -26,7 +26,7 @@ use pbs_api_types::{
 };
 use pbs_config::BackupLockGuard;
 
-use crate::backup_info::{BackupDir, BackupGroup, BackupInfo};
+use crate::backup_info::{BackupDir, BackupGroup, BackupInfo, OLD_LOCKING};
 use crate::chunk_store::ChunkStore;
 use crate::dynamic_index::{DynamicIndexReader, DynamicIndexWriter};
 use crate::fixed_index::{FixedIndexReader, FixedIndexWriter};
@@ -1631,4 +1631,8 @@ impl DataStore {
 
         Ok(())
     }
+
+    pub fn old_locking(&self) -> bool {
+        *OLD_LOCKING
+    }
 }
diff --git a/src/api2/admin/datastore.rs b/src/api2/admin/datastore.rs
index ad11d1fa..39249448 100644
--- a/src/api2/admin/datastore.rs
+++ b/src/api2/admin/datastore.rs
@@ -313,13 +313,23 @@ pub async fn delete_group(
         )?;
 
         let delete_stats = datastore.remove_backup_group(&ns, &group)?;
-        if !delete_stats.all_removed() {
-            if error_on_protected {
-                bail!("group only partially deleted due to protected snapshots");
-            } else {
-                warn!("group only partially deleted due to protected snapshots");
-            }
+
+        let error_msg = if datastore.old_locking() {
+            "could not remove empty groups directories due to old locking mechanism.\n\
+            If you are an admin, please reboot PBS or ensure no old backup job is running anymore, \
+            then remove the file '/run/proxmox-backup/old-locking', and reload all PBS daemons"
+        } else if !delete_stats.all_removed() {
+            "group only partially deleted due to protected snapshots"
+        } else {
+            return Ok(delete_stats);
+        };
+
+        if error_on_protected {
+            bail!(error_msg);
+        } else {
+            warn!(error_msg);
         }
+
         Ok(delete_stats)
     })
     .await?
diff --git a/src/api2/admin/namespace.rs b/src/api2/admin/namespace.rs
index e2a5ccd5..6cf88d89 100644
--- a/src/api2/admin/namespace.rs
+++ b/src/api2/admin/namespace.rs
@@ -167,7 +167,14 @@ pub fn delete_namespace(
     let (removed_all, stats) = datastore.remove_namespace_recursive(&ns, delete_groups)?;
     if !removed_all {
         let err_msg = if delete_groups {
-            "group only partially deleted due to protected snapshots"
+            if datastore.old_locking() {
+                "could not remove empty group directoriess due to old locking mechanism.\n\
+                If you are an admin, please reboot PBS or ensure no old backup job is running \
+                anymore, then remove the file '/run/proxmox-backup/old-locking', and reload all \
+                PBS daemons"
+            } else {
+                "group only partially deleted due to protected snapshots"
+            }
         } else {
             "only partially deleted due to existing groups but `delete-groups` not true"
         };