The application performs 1000+ requests to the same web api at max 10 requests per second.
Quite often it crashes producing the trace below.

There are 10 tasks each calling requestHTTP( url, ... in a loop, the loop body is guarantied never ready within a second.

Any ideas what might be causing this Assert to trigger?

The asserts checks are:

assert(!m_requesting, "Interleaved HTTP client requests detected!");
assert(!m_responding, "Interleaved HTTP client request/response detected!");

Stack trace:

[main(/cjf-fin) FATAL] TaskFiber getting terminated due to an uncaught core.exception.AssertError: core.exception.AssertError@C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(642): Interleaved HTTP client request/response detected!
[main(/cjf-fin) FATAL] ----------------
[main(/cjf-fin) FATAL] 0x00007FF7FB505F1A in d_assert_msg
[main(/cjf-fin) FATAL] 0x00007FF7FB3B4130 in vibe.http.client.HTTPClient.doRequest at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(641)
[main(/cjf-fin) FATAL] 0x00007FF7FB3B1929 in vibe.http.client.HTTPClient.doRequestWithRetry at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(631)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF8D8 in vibe.http.client.HTTPClient.request at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(602)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF4AA in vibe.http.client.requestHTTP at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(81)
[main(/cjf-fin) FATAL] 0x00007FF7FB3AF34E in vibe.http.client.requestHTTP at C:\Users\arjan\AppData\Local\dub\packages\vibe-http\1.2.1\vibe-http\source\vibe\http\client.d(75)