Stan sampling error ("error occurred during calling the sampler; sampling not done")

Question

I am currently trying to perform a bayesian analysis using the following code, with the brms library:

library(brms)

mymodel <- brms::bf(tau ~ a + b*(gmm^c),
                    a ~ 1,
                    b ~ 1,
                    c ~ 1,
                    nl = TRUE)

myprior <- c(prior(uniform(10^-6, 3000), class = 'b', nlpar = 'a', lb = 10^-6, ub = 3000),
             prior(uniform(10^-6, 1000), class = 'b', nlpar = 'b', lb = 10^-6, ub = 1000),
             prior(uniform(10^-6, 1), class = 'b', nlpar = 'c', lb = 10^-6, ub = 1))

myprior$prior

myfit <- brm(formula = mymodel,
                data = dat,
              family = brmsfamily('Gamma'),
               prior = myprior,
             control = list(adapt_delta = 0.95),
                iter = 10000,
              warmup = 5000,
                thin = 5,
              chains = 1,
               cores = 6,
             refresh = 0)

The dat dataframe has four columns, and multiple lines:

> print(dat, n=45)
# A tibble: 14,456 × 4
   fase  ts          gmm   tau
   <fct> <fct>     <dbl> <dbl>
 1 1     30     0.00316   19.3
 2 1     30     0.0153    45.0
 3 1     30     0.0336    55.9
 4 1     30     0.0654    60.5
 5 1     30     0.124     61.7
 6 1     30     0.234     62.4
 7 1     30     0.440     64.0
 8 1     30     0.828     65.2
 9 1     30     1.56      67.3
10 1     30     2.93      71.6
11 1     30     5.51      78.7
12 1     30    10.4       84.6
13 1     30    19.5       91.1
14 1     30    36.8       99.7
15 1     30    69.2      113. 
16 1     30     0.00558   13.8
17 1     30     0.0172    26.4
18 1     30     0.0342    31.5
19 1     30     0.0657    34.3
20 1     30     0.124     34.9
21 1     30     0.234     35.7
22 1     30     0.440     36.7
23 1     30     0.828     38.4
24 1     30     1.56      41.4
25 1     30     2.93      44.0
26 1     30     5.51      49.1
27 1     30    10.4       53.3
28 1     30    19.5       58.2
29 1     30    36.8       65.3
30 1     30    69.2       77.8
31 1     30     0.000369  24.4
32 1     30     0.00252   91.4
33 1     30     0.0194   175. 
34 1     30     0.0599   229  
35 1     30     0.124    240. 
36 1     30     0.232    244. 
37 1     30     0.439    254. 
38 1     30     0.829    256. 
39 1     30     1.56     258. 
40 1     30     2.93     273. 
41 1     30     5.52     289. 
42 1     30    10.4      311. 
43 1     30    19.5      333. 
44 1     30    36.8      346. 
45 1     30    69.2      349. 

When running the code, I get the following error:
[1] "Error : Initialization failed."
[1] "error occurred during calling the sampler; sampling not done"

Does anyone know what could be causing this? Thanks in advance!