Change to case when, just to support if someone change values to something invalid directly in the database, and fixed tes

src/Umbraco.Core/Persistence/SqlSyntax/SqlCeSyntaxProvider.cs

@@ -167,12 +167,12 @@ where table_name=@0 and column_name=@1", tableName, columnName).FirstOrDefault()
             // *not* using a unique 'WHERE IN' query here because the *order* of lockIds is important to avoid deadlocks
             foreach (var lockId in lockIds)
             {
-                var i = db.Execute(@"UPDATE umbracoLock SET value = value*-1 WHERE id=@id", new { id = lockId });
+                var i = db.Execute(@"UPDATE umbracoLock SET value = (CASE WHEN (value=1) THEN -1 ELSE 1 END) WHERE id=@id", new { id = lockId });
                 if (i == 0) // ensure we are actually locking!
                     throw new ArgumentException($"LockObject with id={lockId} does not exist.");
             }
         }
-        
+
         public override void ReadLock(IDatabase db, params int[] lockIds)
         {
             // soon as we get Database, a transaction is started

src/Umbraco.Core/Persistence/SqlSyntax/SqlServerSyntaxProvider.cs

@@ -261,7 +261,7 @@ where tbl.[name]=@0 and col.[name]=@1;", tableName, columnName)
             foreach (var lockId in lockIds)
             {
                 db.Execute(@"SET LOCK_TIMEOUT 1800;");
-                var i = db.Execute(@"UPDATE umbracoLock WITH (REPEATABLEREAD) SET value = value*-1 WHERE id=@id", new { id = lockId });
+                var i = db.Execute(@"UPDATE umbracoLock WITH (REPEATABLEREAD) SET value = (CASE WHEN (value=1) THEN -1 ELSE 1 END) WHERE id=@id", new { id = lockId });
                 if (i == 0) // ensure we are actually locking!
                     throw new ArgumentException($"LockObject with id={lockId} does not exist.");
             }

src/Umbraco.Tests/Persistence/LocksTests.cs

@@ -36,7 +36,7 @@ namespace Umbraco.Tests.Persistence
         {
             using (var scope = ScopeProvider.CreateScope())
             {
-                scope.WriteLock(Constants.Locks.Servers);
+                scope.ReadLock(Constants.Locks.Servers);
                 scope.Complete();
             }
         }