OnlineAccountsManager:

Bump v3 min peer version from 3.2.203 to 3.3.203
No need for toOnlineAccountTimestamp(long) as we only ever use getCurrentOnlineAccountTimestamp().
Latter now returns Long and does the call to NTP.getTime() on behalf of caller, removing duplicated NTP.getTime() calls and null checks in multiple callers.
Add aggregate-signature feature-trigger timestamp threshold checks where needed, near sign() and verify() calls.
Improve logging - but some logging will need to be removed / reduced before merging.

src/main/java/org/qortal/controller/OnlineAccountsManager.java

@@ -128,12 +128,15 @@ public class OnlineAccountsManager {
             return;
 
         byte[] timestampBytes = Longs.toByteArray(onlineAccountsTimestamp);
+        final boolean useAggregateCompatibleSignature = onlineAccountsTimestamp >= BlockChain.getInstance().getAggregateSignatureTimestamp();
 
         Set<OnlineAccountData> replacementAccounts = new HashSet<>();
         for (PrivateKeyAccount onlineAccount : onlineAccounts) {
             // Check mintingAccount is actually reward-share?
 
-            byte[] signature = onlineAccount.sign(timestampBytes);
+            byte[] signature = useAggregateCompatibleSignature
+                    ? Qortal25519Extras.signForAggregation(onlineAccount.getPrivateKey(), timestampBytes)
+                    : onlineAccount.sign(timestampBytes);
             byte[] publicKey = onlineAccount.getPublicKey();
 
             OnlineAccountData ourOnlineAccountData = new OnlineAccountData(onlineAccountsTimestamp, signature, publicKey);
@@ -275,6 +278,8 @@ public class OnlineAccountsManager {
             }
         }
 
+        LOGGER.info(String.format("we have online accounts for timestamps: %s", String.join(", ", this.currentOnlineAccounts.keySet().stream().map(l -> Long.toString(l)).collect(Collectors.joining(", ")))));
+
         return true;
     }
 
@@ -446,6 +451,8 @@ public class OnlineAccountsManager {
      */
     // Block::mint() - only wants online accounts with (online) timestamp that matches block's (online) timestamp so they can be added to new block
     public List<OnlineAccountData> getOnlineAccounts(long onlineTimestamp) {
+        LOGGER.info(String.format("caller's timestamp: %d, our timestamps: %s", onlineTimestamp, String.join(", ", this.currentOnlineAccounts.keySet().stream().map(l -> Long.toString(l)).collect(Collectors.joining(", ")))));
+
         return new ArrayList<>(Set.copyOf(this.currentOnlineAccounts.getOrDefault(onlineTimestamp, Collections.emptySet())));
     }