Hi,
I compared what the manuals say about both switches you connected to each other in order to form a loop intentionally.
And I discovered some difference when considering "STP Port Settings":
In both switches you can do STP port settings for groups (ranges) of ports, but when you do so, switch DGS1210-24P (in contrast to DGS-1510-52) forms a separate Spanning Tree per port group, while switch DGS-1510-52 seems to operate only a single Spanning Tree instance regardless of how many port groups might be configured for this switch.
Hence, if ports 23 and 25 at switch DGS1210-24 happen to belong to different port groups/ranges, a loop might be the result for VLANs defined for both ports (access or trunk).
So, to prevent this, at switch DGS1210-24P you must ascertain that all ports that are connected to other switches (or just all ports 1-24 of the switch) belong to the same STP port group/range.
Just an idea only, may be I'm completely wrong ...
PT